### 2018

• Co-existence of classical snake states and Aharanov-Bohm oscillations along graphene p-n junctions
Peter Makk, Clevin Handschin, Endre Tovari, Kenji Watanabe, Takashi Taniguchi, Klaus Richter, Ming-Hao Liu, and Christian Schönenberger.
Phys. Rev. B, 98:35413, july 2018. arXiv:1804.02590
[Abstract]

Snake states and Aharonov-Bohm interferences are examples of magnetoconductance oscillations that can be observed in a graphene p-n junction. Even though they have already been reported in suspended and encapsulated devices including different geometries, a direct comparison remains challenging as they were observed in separate measurements. Due to the similar experimental signatures of these effects a consistent assignment is difficult, leaving us with an incomplete picture. Here we present measurements on p-n junctions in encapsulated graphene revealing several sets of magnetoconductance oscillations allowing for their direct comparison. We analysed them with respect to their charge carrier density, magnetic field, temperature and bias dependence in order to assign them to either snake states or Aharonov-Bohm oscillations. Furthermore we were able to consistently assign the various Aharonov-Bohm interferences to the corresponding area which the edge states enclose. Surprisingly, we find that snake states and Aharonov-Bohm interferences can co-exist within a limited parameter range

• Wideband and on-chip excitation for dynamical spin injection into graphene
D. I. Indolese, S. Zihlmann, P. Makk, C. Jünger, K. Thodkar, and C. Schönenberger.
submitted, june 2018. arXiv:1806.09356
[Abstract]

Graphene is an ideal material for spin transport as very long spin relaxation times and lengths can be achieved even at room temperature. However, electrical spin injection is challenging due to the conductivity mismatch problem. Spin pumping driven by ferromagnetic resonance is a neat way to circumvent this problem as it produces a pure spin current in the absence of a charge current. Here, we show spin pumping into single layer graphene in micron scale devices. A broadband on-chip RF current line is used to bring micron scale permalloy (Ni80Fe20) pads to ferromagnetic resonance with a magnetic eld tunable resonance condition. At resonance, a spin current is emitted into graphene, which is detected by the inverse spin hall voltage in a close-by platinum electrode. Clear spin current signals are detected down to a power of a few milliwatts over a frequency range of 2 GHz to 8 GHz. This compact device scheme paves the way for more complex device structures and allows the investigation of novel materials.

• Cooper-pair splitting in two parallel InAs nanowires
Shoji Baba, Christian Jünger, Sadashige Matsuo, Andreas Baumgartner, Yosuke Sato, Hiroshi Kamata, Kan Li, Sören Jeppesen, Lars Samuelson, Hongqi Xu, Christian Schönenberger, and Seigo Tarucha.
New Journal of Physics, 20:63021, june 2018. arXiv:1802.08059
[Abstract]

We report on the fabrication and electrical characterization of an InAs double – nanowire (NW) device consisting of two closely placed parallel NWs coupled to a common superconducting electrode on one side and individual normal metal leads on the other. In this new type of device we detect Cooper-pair splitting (CPS) with a sizeable efficiency of correlated currents in both NWs. In contrast to earlier experiments, where CPS was realized in a single NW, demonstrating an intrawire electron pairing mediated by the superconductor (SC), our experiment demonstrates an inter- wire interaction mediated by the common SC. The latter is the key for the realization of zero-magnetic field Majorana bound states, or Parafermions; in NWs and therefore constitutes a milestone towards topological superconductivity. In addition, we observe transport resonances that occur only in the superconducting state, which we tentatively attribute to Andreev Bound states and/or Yu-Shiba resonances that form in the proximitized section of one NW.

• Signatures of van Hove singularities probed by the supercurrent in a graphene – hBN superlattice
D. I. Indolese, R. Delagrange, P. Makk, J. R. Wallbank, K. Wanatabe, T. Taniguchi, and C. Schönenberger.
submitted, may 2018. arXiv:1805.10184
[Abstract]

The bandstructure of graphene can be strongly modified when it is aligned with its Boron Nitride substrate. A moiré superlattice forms, which manifests itself by the appearance of new Dirac points, accompanied by van Hove singularities. In this work, we present supercurrent measurements in a Josephson junction made from such a graphene superlattice in the long and diffusive transport regime, where the supercurrent depends on the Thouless energy. We can then estimate the specific density of states of the graphene superlattice from the combined measurement of the critical current and the normal state resistance. The result matches with theoretical predictions and highlights the strong increase of the density of states at the van Hove singularities. By measuring the magnetic field dependence of the supercurrent, we find the presence of edge currents at these singularities. We explain it by the reduction of the Fermi velocity associated with the flat band at the van Hove singularity, which suppresses the supercurrent in the bulk while the electrons at the edge remain less localized, resulting in an edge supercurrent. We attribute this different behavior of the edges to defects or chemical doping.

• Blocking-state influence on shot noise and conductance in quantum dots
M. -C. Harabula, V. Ranjan, R. Haller, G. Fülöp, and C. Schönenberger.
Phys. Rev. B, 97:115403, mar 2018. arXiv:1801.00286
[Abstract]

Quantum dots (QDs) investigated through electron transport measurements often exhibit varying, state-dependent tunnel couplings to the leads. Under speci c conditions, weakly coupled states can result in a strong suppression of the electrical current and they are correspondingly called blocking states. Using the combination of conductance and shot noise measurements, we investigate blocking states in carbon nanotube (CNT) QDs. We report negative di erential conductance and super- Poissonian noise. The enhanced noise is the signature of electron bunching, which originates from random switches between the strongly and weakly conducting states of the QD. Negative differential conductance appears here when the blocking state is an excited state. In this case, at the threshold voltage where the blocking state becomes populated, the current is reduced. Using a master equation approach, we provide numerical simulations reproducing both the conductance and the shot noise pattern observed in our measurements.

• Large spin relaxation anisotropy and valley-Zeeman spin-orbit coupling in WSe2/Gr/hBN heterostructures
S. Zihlmann, A. W. Cummings, J. H. Garcia, M. Kedves, K. Watanabe, T. Taniguchi, C. Schönenberger, and P. Makk.
Phys. Rev. B, feb 2018. arXiv:1712.05678
[Abstract]

Large spin-orbital proximity effects have been predicted in graphene interfaced with a transition metal dichalcogenide layer. Whereas clear evidence for an enhanced spin-orbit coupling has been found at large carrier densities, the type of spin-orbit coupling and its relaxation mechanism remained unknown. We show for the first time an increased spin-orbit coupling close to the charge neutrality point in graphene, where topological states are expected to appear. Single layer graphene encapsulated between the transition metal dichalcogenide WSe2 and hBN is found to exhibit exceptional quality with mobilities as high as 100 000 cm2/Vs. At the same time clear weak anti-localization indicates strong spin-orbit coupling and a large spin relaxation anisotropy due to the presence of a dominating symmetric spin-orbit coupling is found. Doping dependent measurements show that the spin relaxation of the in-plane spins is largely dominated by a valley-Zeeman spin-orbit coupling and that the intrinsic spin-orbit coupling plays a minor role in spin relaxation. The strong spin-valley coupling opens new possibilities in exploring spin and valley degree of freedoms in graphene with the realization of new concepts in spin manipulation.

• Charge transport in a single molecule transistor probed by scanning tunneling microscopy
S. Bouvron, R. Maurand, A. Graf, P. Erler, L. Gragnaniello, M. Skripnik, D. Wiedmann, C. Engesser, C. Nef, W. Fu, C. Schönenberger, F. Paulya, and M. Fonin.
Nanoscale, 10:1487-1493, jan 2018.
[Abstract]

We report on the scanning tunneling microscopy/spectroscopy (STM/STS) study of cobalt phthalocyanine (CoPc) molecules deposited onto a back-gated graphene device. We observe a clear gate voltage (Vg) dependence of the energy position of the features originating from the molecular states. Based on the analysis of the energy shifts of the molecular features upon tuning Vg, we are able to determine the nature of the electronic states that lead to a gapped differential conductance. Our measurements show that capacitive couplings of comparable strengths exist between the CoPc molecule and the STM tip as well as between CoPc and graphene, thus facilitating electronic transport involving only unoccupied molecular states for both tunneling bias polarities. These findings provide novel information on the interaction between graphene and organic molecules and are of importance for further studies, which envisage the realization of single molecule transistors with non-metallic electrodes.

• High-detection efficiency and low-timing jitter with amorphous superconducting nanowire single-photon detectors
M. Caloz, M. Perrenoud, C. Autebert, B. Korzh, M. Weiss, C. Schönenberger, R. J. Warburton, H. Zbinden, and F. Bussières.
Appl. Phys. Lett., 112:61103, jan 2018. arXiv:1710.06740
[Abstract]

Recent progress in the development of superconducting nanowire single-photon detectors (SNSPDs) made of amorphous material has delivered excellent performances, and has had a great impact on a range of research fields. Despite showing the highest system detection effciency (SDE) ever reported with SNSPDs, amorphous materials typically lead to lower critical currents, which impacts on their jitter performance. Combining a very low jitter and a high SDE remains a challenge. Here, we report on highly effcient superconducting nanowire single-photon detectors based on amorphous MoSi, combining system jitters as low as 26 ps and a SDE of 80\% at 1550 nm. We also report detailed observations on the jitter behaviour, which hints at intrinsic limitations and leads to practical implications for SNSPD performance.

• Quantum-Confined Stark Effect in a MoS2 Monolayer van der Waals Heterostructure
J. G. Roch, N. Leisgang, G. Froehlicher, P. Makk, K. Watanabe, T. Taniguchi, C. Schönenberger, and R. J. Warburton.
Nano Letters, 18:1070−1074, jan 2018. arXiv:1710.09750
[Abstract]

The optics of dangling-bond-free van der Waals heterostructures containing transition metal dichalcogenides are dominated by excitons. A crucial property of a confined exciton is the quantum confined Stark effect (QCSE). Here, such a heterostructure is used to probe the QCSE by applying a uniform vertical electric field across a molybdenum disulfide (MoS2) monolayer. The photoluminescence emission energies of the neutral and charged excitons shift quadratically with the applied electric field, provided that the electron density remains constant, demonstrating that the exciton can be polarized. Stark shifts corresponding to about half the homogeneous linewidth were achieved. Neutral and charged exciton polarizabilities of (7.8 ± 1.0) × 10−10 and (6.4 ± 0.9) × 10−10 D m V−1 at relatively low electron density (~10^12 cm−2) have been extracted, respectively. These values are one order of magnitude lower than the previously reported values but in line with theoretical calculations. The methodology presented here is versatile and can be applied to other semiconducting layered materials.

• Spin transport in two-layer-CVD-hBN/graphene/hBN heterostructures
M. Gurram, S. Omar, S. Zihlmann, P. Makk, Q. C. Li, Y. F. Zhang, C. Schönenberger, and B. J. van Wees.
Phys. Rev. B, 97:45411, jan 2018. arXiv:1712.00815
[Abstract]

We study room-temperature spin transport in graphene devices encapsulated between a layer-by-layer-stacked two-layer-thick chemical vapor deposition (CVD) grown hexagonal boron nitride (hBN) tunnel barrier, and a few-layer-thick exfoliated-hBN substrate. We find mobilities and spin-relaxation times comparable to that of SiO2 substrate-based graphene devices, and we obtain a similar order of magnitude of spin relaxation rates for both the Elliott-Yafet and D’Yakonov-Perel’ mechanisms. The behavior of ferromagnet/two-layer-CVDhBN/ graphene/hBN contacts ranges from transparent to tunneling due to inhomogeneities in the CVD-hBN barriers. Surprisingly, we find both positive and negative spin polarizations for high-resistance two-layer-CVDhBN barrier contacts with respect to the low-resistance contacts. Furthermore, we find that the differential spininjection polarization of the high-resistance contacts can be modulated by dc bias from −0.3 to +0.3 V with no change in its sign, while its magnitude increases at higher negative bias. These features point to the distinctive spin-injection nature of the two-layer-CVD-hBN compared to the bilayer-exfoliated-hBN tunnel barriers.

### 2017

• Andreev bound states probed in three-terminal quantum dots
J. Gramich, A. Baumgartner, and C. Schönenberger.
Phys. Rev. B, 96:195418, nov 2017. arXiv:1612.01201
[Abstract]

Andreev bound states (ABSs) are well-de ned many-body quantum states that emerge from the hybridization of individual quantum dot (QD) states with a superconductor and exhibit very rich and fundamental phenomena. We demonstrate several new electron transport phenomena mediated by ABSs that form on three-terminal carbon nanotube (CNT) QDs, with one superconducting (S) contact in the center and two adjacent normal metal (N) contacts. Three-terminal spectroscopy allows us to identify the coupling to the N contacts as the origin of the Andreev resonance (AR) linewidths and to determine the critical coupling strengths to S, for which a ground state (or quantum phase) transition in such S-QD systems can occur. In addition, we ascribe replicas of the lowest-energy ABS resonance to transitions between the ABS and odd-parity excited QD states, a process we call excited state ABS resonances. In the conductance between the two N contacts we find a characteristic pattern of positive and negative differential subgap conductance, which we explain by considering two nonlocal processes, the creation of Cooper pairs in S by electrons from both N terminals, and a novel transport mechanism called resonant ABS tunneling, possible only in multi-terminal QD devices. In the latter process, electrons are transferred via the ABS without effectively creating Cooper pairs in S. The three-terminal geometry also allows spectroscopy experiments with different boundary conditions, for example by leaving S floating. Surprisingly, we find that, depending on the boundary conditions and the device parameters, the experiments either show single-particle Coulomb blockade resonances, ABS characteristics, or both in the same measurements, seemingly contradicting the notion of ABSs replacing the single particle states as eigenstates of the QD. We qualitatively explain these results as originating from the nite time scale required for the coherent oscillations between the superposition states after a single electron tunneling event. These experiments demonstrate that three-terminal experiments on a single complex quantum object can also be useful to investigate charge dynamics otherwise not accessible due to the very high frequencies.

• Measuring a Quantum Dot with an Impedance-Matching On-Chip Superconducting LC Resonator at Gigahertz Frequencies
M. -C. Harabula, T. Hasler, G. Fülöp, M. Jung, V. Ranjan, and C. Schönenberger.
Phys. Rev. Appl., 8:54006, nov 2017. arXiv:1707.09061
[Abstract]

We report on the realization of a bonded-bridge on-chip superconducting coil and its use in impedance matching a highly ohmic quantum dot (QD) to a 3-GHz measurement setup. The coil, modeled as a lumped-element LC resonator, is more compact and has a wider bandwidth than resonators based on coplanar transmission lines (e.g., λ/4 impedance transformers and stub tuners), at potentially better signal-to-noise ratios. Specifically, for measurements of radiation emitted by the device, such as shot noise, the 50 × larger bandwidth reduces the time to acquire the spectral density. The resonance frequency, close to 3.25 GHz, is 3 times higher than that of the one previously reported, a wire-bonded coil. As a proof of principle, we fabricate an LC circuit that achieves impedance matching to an approximately 15 kOhm load and validate it with a load defined by a carbon nanotube QD, whose shot noise we measure in the Coulomb-blockade regime.

• Giant Valley-Isospin Conductance Oscillations in Ballistic Graphene
C. Handschin, P.Makk, P. Rickhaus, R. Maurand, K. Watanabe, T. Taniguchi, K. Richter, Ming-Hao Liu, and C. Schönenberger.
Nano Letters, 17:5389-5393, aug 2017. arXiv:1708.09614
[Abstract]

At high magnetic fields the conductance of graphene is governed by the half-integer quantum Hall effect. By local electrostatic gating a p−n junction perpendicular to the graphene edges can be formed, along which quantum Hall channels copropagate. It has been predicted by Tworzidło and co-workers that if only the lowest Landau level is filled on both sides of the junction, the conductance is determined by the valley (isospin) polarization at the edges and by the width of the flake. This effect remained hidden so far due to scattering between the channels copropagating along the p−n interface (equilibration). Here we investigate p−n junctions in encapsulated graphene with a movable p−n interface with which we are able to probe the edge configuration of graphene flakes. We observe large quantum conductance oscillations on the order of e2/h which solely depend on the p−n junction position providing the first signature of isospin-defined conductance. Our experiments are underlined by quantum transport calculations.

• Restoring the Electrical Properties of CVD Graphene via Physisorption of Molecular Adsorbates
K. Thodkar, D-. Thompson, F. Lüönd, L. Moser, F. Overney, L. Marot, C. Schönenberger, B. Jeanneret, and M. Calame.
ACS Appl. Mater. Interfaces, 9(29):25014-25022, july 2017.
[Abstract]

Chemical vapor deposition (CVD) is a powerful technique to produce graphene for large-scale applications. Polymer-assisted wet transfer is commonly used to move the graphene onto silicon substrates, but the resulting devices tend to exhibit p-doping, which decreases the device quality and reproducibility. In an effort to better understand the origin of this effect, we coated graphene with n-methyl-2-pyrrolidone (NMP) and hexamethyldisilazane (HMDS) molecules that exhibit negligible charge transfer to graphene but bind more strongly to graphene than ambient adsorbents. Using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), electrical transport measurements, and quantum mechanical computer simulations, we show that the molecules help in the removal of p-doping, and our data indicate that the molecules do this by replacing ambient adsorbents (typically O2 and water) on the graphene surface. This very simple method of improving the electronic properties of CVD graphene by passivating its surface with common solvent molecules will accelerate the development of CVD graphene-based devices

• Contactless Microwave Characterization of Encapsulated Graphene p-n Junctions
V. Ranjan, S. Zihlmann, P. Makk, K. Watanabe, T. Taniguchi, and C. Schönenberger.
Phys. Rev. Applied, 7(5):54015, may 2017. arXiv:1702.02071
[Abstract]

Accessing intrinsic properties of a graphene device can be hindered by the influence of contact electrodes. Here, we capacitively couple graphene devices to superconducting resonant circuits and observe clear changes in the resonance-frequency and -widths originating from the internal charge dynamics of graphene. This allows us to extract the density of states and charge relaxation resistance in graphene p-n junctions without the need of electrical contacts. The presented characterizations pave a fast, sensitive and non-invasive measurement of graphene nanocircuits.

• Optically probing the detection mechanism in a molybdenum silicide superconducting nanowire single-photon detector
M. Caloz, B. Korzh, N. Timoney, M. Weiss, S. Gariglio, R. J. Warburton, C. Schönenberger, J. Renema, H. Zbinden, and F. Bussieres.
Applied Physics Letters, 110(8):83106, feb 2017. arXiv:1611.08238
[Abstract]

We experimentally investigate the detection mechanism in a meandered molybdenum silicide superconducting nanowire single-photon detector by characterising the detection probability as a function of bias current in the wavelength range of 750–2050 Onm. Contrary to some previous observations on niobium nitride or tungsten silicide detectors, we find that the energy-current relation is nonlinear in this range. Furthermore, thanks to the presence of a saturated detection efficiency over the whole range of wavelengths, we precisely quantify the shape of the curves. This allows a detailed study of their features, which are indicative of both Fano fluctuations and position-dependent effects.

• Fabry-Pérot Resonances in a Graphene/hBN Moiré Superlattice
C. Handschin, P.Makk, P. Rickhaus, M. -H. Liu, K. Watanabe, T. Taniguchi, K. Richter, and C. Schönenberger.
Nano Letters, 17:328-333, jan 2017. arXiv:1701.09141
[Abstract]

While Fabry-Pérot (FP) resonances and Moiré superlattices are intensively studied in graphene on hexagonal boron nitride (hBN), the two effects have not been discussed in their coexistence. Here we investigate the FP oscillations in a ballistic pnp-junctions in the presence and absence of a Moiré superlattice. First, we address the effect of the smoothness of the confining potential on the visibility of the FP resonances and carefully map the evolution of the FP cavity size as a function of densities inside and outside the cavity in the absence of a superlattice, when the cavity is bound by regular pn-junctions. Using a sample with a Moiré superlattice, we next show that an FP cavity can also be formed by interfaces that mimic a pn-junction but are defined through a satellite Dirac point due to the superlattice. We carefully analyze the FP resonances, which can provide insight into the band-reconstruction due to the superlattice.

• Charge Noise in Organic Electrochemical Transistors
R. L. Stoop, K. Thodkar, M. Sessolo, H. J. Bolink, and Calame M. C. Schönenberger.
Phys. Rev. Appl., 7(1):14009, jan 2017.
[Abstract]

Organic electrochemical transistors (OECTs) are increasingly studied as transducers in sensing applications. While much emphasis has been placed on analyzing and maximizing the OECT signal, noise has been mostly ignored, although it determines the resolution of the sensor. The major contribution to the noise in sensing devices is the 1/f noise, dominant at low frequency. In this work, we demonstrate that the 1/f noise in OECTs follows a charge-noise model, which reveals that the noise is due to charge fuctuations in proximity or within the bulk of the channel material. We present the noise scaling behavior with gate voltage, channel dimensions and polymer thickness. Our results suggest the use of large area channels in order to maximize the signal-to-noise-ratio (SNR) for biochemical and electrostatic sensing applications. Comparison with literature shows that the magnitude of the noise in OECTs is similar to that observed in graphene transistors, and only slightly higher compared to Carbon nanotubes and Silicon nanowire devices. In a model ion-sensing experiment with OECTs, we estimate crucial parameters such as the characteristic SNR and corresponding limit of detection.

### 2016

• Gate-controlled conductance enhancement from quantum Hall channels along graphene p-n junctions
E. Tovari, P. Makk, Ming-Hao Liu, P. Rickhaus, Z. Kovas-Krausz, C. Schönenberger, and S. Csonka.
Nanoscale, 8(47):19910-19916, dec 2016. arXiv:1606.08007
[Abstract]

The formation of quantum Hall channels inside the bulk of graphene is studied using various contact and gate geometries. p-n junctions are created along the longitudinal direction of samples, and enhanced conductance is observed in the case of bipolar doping due to the new conducting channels formed in the bulk, whose position, propagating direction and, in one geometry, coupling to electrodes are determined by the gate-controlled filling factor across the device. This effect could be exploited to probe the behavior and interaction of quantum Hall channels protected against uncontrolled scattering at the edges.

• Microwave Photodetection in an Ultraclean Suspended Bilayer Graphene p–n Junction
M. Jung, P. Rickhaus, S. Zihlmann, P. Makk, and C. Schönenberger.
Nano Letters, 16(11):6988-6993, nov 2016. arXiv:1702.01529
[Abstract]

We explore the potential of bilayer graphene as a cryogenic microwave photodetector by studying the microwave absorption in fully suspended clean bilayer graphene p–n junctions in the frequency range of 1–5 GHz at a temperature of 8 K. We observe a distinct photocurrent signal if the device is gated into the p–n regime, while there is almost no signal for unipolar doping in either the n–n or p–p regimes. Most surprisingly, the photocurrent strongly peaks when one side of the junction is gated to the Dirac point (charge-neutrality point CNP), while the other remains in a highly doped state. This is different to previous results where optical radiation was used. We propose a new mechanism based on the phototermal effect explaining the large signal. It requires contact doping and a distinctly different transport mechanism on both sides: one side of graphene is ballistic and the other diffusive. By engineering partially diffusive and partially ballistic devices, the photocurrent can drastically be enhanced.

• Magnetoresistance engineering and singlet/triplet switching in InAs nanowire quantum dots with ferromagnetic sidegates
G. Fábián, P. Makk, M. H. Madsen, J. Nygård, C. Schönenberger, and A. Baumgartner.
Physical Review B, 94(19):195415, nov 2016. arXiv:1608.07143
[Abstract]

We present magnetoresistance (MR) experiments on an InAs nanowire quantum dot device with two ferromagnetic sidegates (FSGs) in a split-gate geometry. The wire segment can be electrically tuned to a single dot or to a double dot regime using the FSGs and a backgate. In both regimes we find a strong MR and a sharp MR switching of up to 25\% at the field at which the magnetizations of the FSGs are inverted by the external field. The sign and amplitude of the MR and the MR switching can both be tuned electrically by the FSGs. In a double dot regime close to pinch-off we find {\it two} sharp transitions in the conductance, reminiscent of tunneling MR (TMR) between two ferromagnetic contacts, with one transition near zero and one at the FSG switching fields. These surprisingly rich characteristics we explain in several simple resonant tunneling models. For example, the TMR-like MR can be understood as a stray-field controlled transition between singlet and a triplet double dot states. Such local magnetic fields are the key elements in various proposals to engineer novel states of matter and may be used for testing electron spin-based Bell inequalities.

• A success story
Christel Möller and Christian Schönenberger.
Nature Nanotechnology, 11(10):908, oct 2016.

• Comparative study of single and multi domain CVD graphene using large-area Raman mapping and electrical transport characterization
K. Thodkar, El M. Abbassi, F. Lüönd, F. Overney, C. Schönenberger, B. Jeanneret, and M. Calame.
physica status solidi (RRL) – Rapid Research Letters, 10(11):807-811, sep 2016.
[Abstract]

We systematically investigate the impact of granularity in CVD graphene films by performing Raman mapping and electrical characterization of single (SD) and multi domain (MD) graphene. In order to elucidate the quality of the graphene film, we study its regional variations using large-area Raman mapping and compare the G and 2D peak positions of as-transferred chemical vapor deposited (CVD) graphene on SiO2 substrate. We find a similar upshift in wavenumber in both SD and MD graphene in comparison to freshly exfoliated graphene. In our case, doping could play the dominant role behind the observation of such upshifts rather than the influence due to strain. Interestingly, the impact of the polymer-assisted wet transfer process is the same in both the CVD graphene types. The electrical characterization shows that SD graphene exhibits a substantially higher (a factor 5) field-effect mobility when compared to MD graphene. We attribute the low sheet resistance and mobility enhancement to a decrease in charge carrier scattering thanks to a reduction of the number of grain boundaries and defects in SD graphene.

• Full characterization of a carbon nanotube parallel double quantum dot
G. Abulizi, A. Baumgartner, and C. Schönenberger.
Physica Status Solidi B, 253(12):2428-2432, aug 2016. arXiv:1605.02300v1
[Abstract]

We have measured the differential conductance of a parallel carbon nanotube (CNT) double quantum dot (DQD) with strong inter-dot capacitance and inter-dot tunnel coupling. Nominally, the device consists of a single CNT with two contacts. However, we identify two sets of Coulomb blockade (CB) diamonds that do not block transport individually, which suggests that two quantum dots (QDs) are contacted in parallel. We find strong and periodic anti-crossings in the gate and bias dependence, which are only possible if the QDs have similar characteristics. We discuss qualitatively the level spectrum and the involved transport processes in this device and extract the DQD coupling parameters. These results lead us to believe that clean and undoped QDs are formed parallel to the CNT axis, possibly on the outer and inner shells of a multi-wall CNT, or in a double-stranded CNT bundle.

• Label-Free FimH Protein Interaction Analysis Using Silicon Nanoribbon BioFETs
M. Wipf, R. L. Stoop, G. Navarra, S. Rabbani, B. Ernst, K. Bedner, C. Schönenberger, and M. Calame.
ACS Sensors, 1(6):781-788, jun 2016.
[Abstract]

The detection of biomarkers at very low concentration and low cost is increasingly important for clinical diagnosis. Moreover, monitoring affinities for receptorantagonist interactions by time-resolved measurements is crucial for drug discovery and development. Biosensors based on ion-sensitive field-effect transistors (BioFETs) are promising candidates for being integrated into CMOS structures and cost-effective production. The detection of DNA and proteins with silicon nanowires has been successfully demonstrated using high affinity systems such as the biotin−streptavidin interaction. Here, we show the time-resolved label-free detection of the interaction of the bacterial FimH lectin with an immobilized mannose ligand on gold-coated silicon nanoribbon BioFETs. By comparing our results with a commercial state of the art surface plasmon resonance system, additional surface effects become visible when using this charge based detection method. Furthermore, we demonstrate the effect of sensor area on signal-to-noise ratio and estimate the theoretical limit of detection.

• Implementing Silicon Nanoribbon Field-Effect Transistors as Arrays for Multiple Ion Detection
R. Stoop, M. Wipf, S. Müller, K. Bedner, I. A. Wright, C. J. Martin, E. C. Constable, A. Fanget, C. Schönenberger, and M. Calame.
Biosensors, 6(2):21, may 2016.
[Abstract]

Ionic gradients play a crucial role in the physiology of the human body, ranging from metabolism in cells to muscle contractions or brain activities. To monitor these ions, inexpensive, label-free chemical sensing devices are needed. Field-effect transistors (FETs) based on silicon (Si) nanowires or nanoribbons (NRs) have a great potential as future biochemical sensors as they allow for the integration in microscopic devices at low production costs. Integrating NRs in dense arrays on a single chip expands the field of applications to implantable electrodes or multifunctional chemical sensing platforms. Ideally, such a platform is capable of detecting numerous species in a complex analyte. Here, we demonstrate the basis for simultaneous sodium and fluoride ion detection with a single sensor chip consisting of arrays of gold-coated SiNR FETs. A microfluidic system with individual channels allows modifying the NR surfaces with self-assembled monolayers of two types of ion receptors sensitive to sodium and fluoride ions. The functionalization procedure results in a differential setup having active fluoride- and sodium-sensitive NRs together with bare gold control NRs on the same chip. Comparing functionalized NRs with control NRs allows the compensation of non-specific contributions from changes in the background electrolyte concentration and reveals the response to the targeted species.

• Signatures of single quantum dots in graphene nanoribbons within the quantum Hall regime
E. Tovari, P. Makk, P. Rickhaus, C. Schönenberger, and S. Csonka.
Nanoscale, 8(22):11480-11486, may 2016. arXiv:1601.01628
[Abstract]

We report on the observation of periodic conductance oscillations near quantum Hall plateaus in suspended graphene nanoribbons. They are attributed to single quantum dots that form in the narrowest part of the ribbon, in the valleys and hills of a disorder potential. In a wide flake with two gates, a double-dot system’s signature has been observed. Electrostatic confinement is enabled in single-layer graphene due to the gaps that form between Landau levels, suggesting a way to create gate-defined quantum dots that can be accessed with quantum Hall edge states.

• Wet etch methods for InAs nanowire patterning and self-aligned electrical contacts
G. Fülöp, S. d’Hollosy, L. Hofstetter, A. Baumgartner, J. Nygard, C. Schönenberger, and S. Csonka.
Nanotechnology, 27(19):195303, apr 2016. arXiv:1601.01562
[Abstract]

Advanced synthesis of semiconductor nanowires (NWs) enables their application in diverse fields, notably in chemical and electrical sensing, photovoltaics, or quantum electronic devices. In particular, indium arsenide (InAs) NWs are an ideal platform for quantum devices, e.g. they may host topological Majorana states. While the synthesis has been continously perfected, only a few techniques have been developed to tailor individual NWs after growth. Here we present three wet chemical etch methods for the post-growth morphological engineering of InAs NWs on the sub-100 nm scale. The first two methods allow the formation of self-aligned electrical contacts to etched NWs, while the third method results in conical shaped NW profiles ideal for creating smooth electrical potential gradients and shallow barriers. Low temperature experiments show that NWs with etched segments have stable transport characteristics and can serve as building blocks of quantum electronic devices. As an example we report the formation of a single electrically stable quantum dot between two etched NW segments.

• Subgap resonant quasiparticle transport in normal-superconductor quantum dot devices
J. Gramich, A. Baumgartner, and C. Schönenberger.
Applied Physics Letters, 108(17):172604, apr 2016. arXiv:1601.00672
[Abstract]

We report thermally activated transport resonances for biases below the superconducting energy gap in a carbon nanotube quantum dot (QD) device with a superconducting Pb and a normal metal contact. These resonances are due to the superconductor’s finite quasi-particle population at elevated temperatures and can only be observed when the QD life-time broadening is considerably smaller than the gap. This condition is fulfilled in our QD devices with optimized Pd/Pb/In multi-layer contacts, which result in reproducibly large and “clean” superconducting transport gaps with a strong conductance suppression for subgap biases. We show that these gaps close monotonically with increasing magnetic field and temperature. The accurate description of the subgap resonances by a simple resonant tunneling model illustrates the ideal characteristics of the reported Pb contacts and gives an alternative access to the tunnel coupling strengths in a QD.

• Cooper-Paare tunneln durch einen Quantenpunkt
Andreas Baumgartner, Jörg Gramich, and Christian Schönenberger.
Physik in unserer Zeit, 47(2):62-63, mar 2016.
[Abstract]

Elektronische Bauteile aus Supraleitern und Quantenpunkten zeigen eine Vielzahl von neuen und fundamentalen physikalischen Eigenschaften und stellen neue quantentechnologische Anwendungen in Aussicht. Kuerzlich ist es gelungen, den wohl grundlegendsten Transportprozess in einer solchen Struktur in Experimenten zu identifizieren, naemlich den direkten Transport von Elektronen aus einem Supraleiter durch einen Quantenpunkt, das sogenannte Andreev-Tunneln. Das Verstaendnis dieses Prozesses liefert die Grundlage fuer zukuenftige Anwendungen, die quantenmechanische Phaenomene in elektronischen Bauteilen ausnutzen werden.

• Additional peak appearing in the one-photon luminescence of single gold nanorods
T. Fröhlich, C. Schönenberger, and M. Calame.
Optics Letters, 41(7):1325, mar 2016.
[Abstract]

We used a confocal laser microscope to investigate the one photon photoluminescence (PL) of gold antennas. The PL spectra can be precisely fitted to a plasmon-enhanced PL model. For increasing the antenna length, the energy peak position decreases continuously until it reaches a value of 1.7–1.8 eV. For longer antennas and smaller plasmon energies, we observe an additional, persistent shoulder in the PL spectra, which we explain by a Gaussian-shaped peak at ΔX ≈ 1.78–1.79 eV. We attribute this behavior to the opening of an additional decay path for electrons at the gold interband transition edge, which we observe only for long antennas.

• Spin transport in fully hexagonal boron nitride encapsulated graphene
M. Gurram, S. Omar, S. Zihlmann, P. Makk, C. Schönenberger, and B. J. van Wees.
Physical Review B, 93(11):115441, mar 2016. arXiv:1603.04357
[Abstract]

We study fully hexagonal boron nitride (hBN) encapsulated graphene spin valve devices at room temperature. The device consists of a graphene channel encapsulated between two crystalline hBN flakes: thick-hBN flake as a bottom gate dielectric substrate which masks the charge impurities from Si_{O2}/Si substrate and single-layer thin-hBN flake as a tunnel barrier. Full encapsulation prevents the graphene from coming in contact with any polymer/chemical during the lithography and thus gives homogeneous charge and spin transport properties across different regions of the encapsulated graphene. Further, even with the multiple electrodes in-between the injection and the detection electrodes which are in conductivity mismatch regime, we observe spin transport over 12.5 $\mu$m-long distance under the thin-hBN encapsulated graphene channel, demonstrating the clean interface and the pinhole-free nature of the thin hBN as an efficient tunnel barrier.

• Role of hexagonal boron nitride in protecting ferromagnetic anostructures from oxidation
S. Zihlmann, P. Makk, C. A. F. Vaz, and C. Schönenberger.
2D Materials, 3(1):11008, feb 2016. arXiv:1509.03087
[Abstract]

Ferromagnetic contacts are widely used to inject spin polarized currents into non-magnetic materials such as semiconductors or 2-dimensional materials like graphene. In these systems, oxidation of the ferromagnetic materials poses an intrinsic limitation on device performance. Here we investigate the role of ex situ transferred chemical vapour deposited hexagonal boron nitride (hBN) as an oxidation barrier for nanostructured cobalt and permalloy electrodes. The chemical state of the ferromagnets was investigated using x-ray photoemission electron microscopy because of its high sensitivity and lateral resolution. We have compared the oxide thickness formed on ferromagnetic nanostructures covered by hBN to uncovered reference structures. Our results show that hBN reduces the oxidation rate of ferromagnetic nanostructures suggesting that it could be used as an ultra-thin protection layer in future spintronic devices.

### 2015

• Competing surface reactions limiting the performance of ion-sensitive field-effect transistors
R. L. Stoop, M. Wipf, S. Müller, K. Bedner, I. A. Wright, C. J. Martin, E. C. Constable, Wangyang Fu, A. Tarasova, M. Calame, and C. Schönenberger.
Sensors and Actuators B, 220:500-507, dec 2015.
[Abstract]

Ion-sensitive field-effect transistors based on silicon nanowires are promising candidates for the detection of chemical and biochemical species. These devices have been established as pH sensors thanks to the large number of surface hydroxyl groups at the gate dielectrics which makes them intrinsically sensitive to protons. To specifically detect species other than protons, the sensor surface needs to be modified. However, the remaining hydroxyl groups after functionalization may still limit the sensor response to the targeted species. Here, we describe the influence of competing reactions on the measured response using a general site-binding model. We investigate the key features of the model with a real sensing example based on gold-coated nanoribbons functionalized with a self-assembled monolayer of calcium-sensitive molecules. We identify the residual pH response as the key parameter limiting the sensor response. The competing effect of pH or any other relevant reaction at the sensor surface has therefore to be included to quantitatively understand the sensor response and prevent misleading interpretations.

• Gate tuneable beamsplitter in ballistic graphene
P. Rickhaus, P. Makk, M. -H. Liu, K. Richter, and C. Schönenberger.
Applied Physics Letters, 107(25):251901, dec 2015. arXiv:1511.03044
[Abstract]

We present a beam splitter in a suspended, ballistic, multiterminal, bilayer graphene device. By using local bottomgates, a p-n interface tilted with respect to the current direction can be formed. We show that the p-n interface acts as a semi-transparent mirror in the bipolar regime and that the reflectance and transmittance of the p-n interface can be tuned by the gate voltages. Moreover, by studying the conductance features appearing in magnetic field, we demonstrate that the position of the p-n interface can be moved by 1μm. The herein presented beamsplitter device can form the basis of electron-optic interferometers in graphene

• Point contacts in encapsulated graphene
C. Handschin, B. Fülöp, P. Makk, S. Blanter, M. Weiss, K. Watanabe, T. Taniguchi, S. Csonka, and C. Schönenberger.
Applied Physics Letters, 107(18):183108, nov 2015. arXiv:1509.04137v1.pdf
[Abstract]

We present a method to establish inner point contacts on hexagonal boron nitride (hBN) encapsulated graphene heterostructures with dimensions as small as 100 nm by pre-patterning the top-hBN in a separate step prior to dry-stacking. 2 and 4-terminal field effect measurements between different lead combinations are in qualitative agreement with an electrostatic model assuming point-like contacts. The measured contact resistances are 0.5-1.5 k$\Omega$ per contact, which is quite low for such small contacts. By applying a perpendicular magnetic fields, an insulating behaviour in the quantum Hall regime was observed, as expected for inner contacts. The fabricated contacts are compatible with high mobility graphene structures and open up the field for the realization of several electron optical proposals.

• Electron optics: Turn the other way
Péter Makk.
Nature Physics, 11(11):894-895, nov 2015.

• Shot Noise of a Quantum Dot Measured with Gigahertz Impedance Matching
T. Hasler, M. Jung, V. Ranjan, G. Puebla-Hellmann, A. Wallraff, and C. Schönenberger.
Physical Review Applied, 4(5):54002, nov 2015. arXiv:1507.04884.pdf
[Abstract]

The demand for a fast high-frequency read-out of high-impedance devices, such as quantum dots, necessitates impedance matching. Here we use a resonant impedance-matching circuit (a stub tuner) realized by on-chip superconducting transmission lines to measure the electronic shot noise of a carbonnanotube quantum dot at a frequency close to 3 GHz in an efficient way. As compared to wideband detection without impedance matching, the signal-to-noise ratio can be enhanced by as much as a factor of 800 for a device with an impedance of 100 kOmega. The advantage of the stub resonator concept is the ease with which the response of the circuit can be predicted, designed, and fabricated. We further demonstrate that all relevant matching circuit parameters can reliably be deduced from power-reflectance measurements and then used to predict the power-transmission function from the device through the circuit. The shot noise of the carbon-nanotube quantum dot in the Coulomb blockade regime shows an oscillating suppression below the Schottky value of 2eI, as well as an enhancement in specific regions

• Ordered nanoparticle arrays interconnected by molecular linkers: electronic and optoelectronic properties
J. Liao, S. Blok, S. J. van der Molen, S. Diefenbach, A. W. Holleitner, C. Schönenberger, A. Vladyka, and M. Calame.
Chem. Soc. Rev., 44(4):999-1014, nov 2015.
[Abstract]

Arrays of metal nanoparticles in an organic matrix have attracted a lot of interest due to their diverse electronic and optoelectronic properties. Recent work demonstrates that nanoparticle arrays can be utilized as a template structure to incorporate single molecules. In this arrangement, the nanoparticles act as electronic contacts to the molecules. By varying parameters such as the nanoparticle material, the matrix material, the nanoparticle size, and the interparticle distance, the electronic behavior of the nanoparticle arrays can be substantially tuned and controlled. Furthermore, via the excitation of surface plasmon polaritons, the nanoparticles can be optically excited and electronically read-out. The versatility and possible applications of well-ordered nanoparticle arrays has been demonstrated by the realization of switching devices triggered optically or chemically and by the demonstration of chemical and mechanical sensing. Interestingly, hexagonal nanoparticle arrays may also become a useful platform to study the physics of collective plasmon resonances that can be described as Dirac-like bosonic excitations.

• Resonant and Inelastic Andreev Tunneling Observed on a Carbon Nanotube Quantum Dot
J. Gramich, A. Baumgartner, and C. Schönenberger.
Physical Review Letters, 115(21):216801, nov 2015. arXiv:1507.00526
[Abstract]

We report the observation of two fundamental sub-gap transport processes through a quantum dot (QD) with a superconducting contact. The device consists of a carbon nanotube contacted by a Nb superconducting and a normal metal contact. First, we find a single resonance with position, shape and amplitude consistent with the theoretically predicted resonant Andreev tunneling (AT) through a single QD level. Second, we observe a series of discrete replicas of resonant AT at a separation of ∼145μeV, with a gate, bias and temperature dependence characteristic for boson-assisted, inelastic AT, in which energy is exchanged between a bosonic bath and the electrons. The magnetic field dependence of the replica’s amplitudes and energies suggest that two different bosons couple to the tunnel process.

• Magnetic Field Tuning and Quantum Interference in a Cooper Pair Splitter
G. Fülöp, F. Domínguez, S. d’Hollosy, A. Baumgartner, P. Makk, M. H. Madsen, V. A. Guzenko, J. Nygard, C. Schönenberger, Levy A. Yeyati, Csonka S. -. in cooperation with the Csonka(Budapest), and Levi Yeyati group (Madrid).
Physical Review Letters, 115(22):227003, nov 2015. arXiv:1507.01036
[Abstract]

Cooper pair splitting (CPS) is a process in which the electrons of naturally occurring spin-singlet pairs in a superconductor are spatially separated using two quantum dots. Here we investigate the evolution of the conductance correlations in an InAs CPS device in the presence of an external magnetic field. In our experiments the gate dependence of the signal that depends on both quantum dots continuously evolves from a slightly asymmetric Lorentzian to a strongly asymmetric Fano-type resonance with increasing field. These experiments can be understood in a simple three – site model, which shows that the nonlocal CPS leads to symmetric line shapes, while the local transport processes can exhibit an asymmetric shape due to quantum interference. These findings demonstrate that the electrons from a Cooper pair splitter can propagate coherently after their emission from the superconductor and how a magnetic field can be used to optimize the performance of a CPS device. In addition, the model calculations suggest that the estimate of the CPS efficiency in the experiments is a lower bound for the actual efficiency.

• Ordered nanoparticle arrays interconnected by molecular linkers: electronic and optoelectronic properties
J. Liao, S. Blok, S. J. van der Molen, S. Diefenbach, A. W. Holleitner, C. Schönenberger, A. Vladykae, and M. Calame.
Chem. Soc. Rev., 44(1):382, nov 2015.
[Abstract]

Arrays of metal nanoparticles in an organic matrix have attracted a lot of interest due to their diverse electronic and optoelectronic properties. Recent work demonstrates that nanoparticle arrays can be utilized as a template structure to incorporate single molecules. In this arrangement, the nanoparticles act as electronic contacts to the molecules. By varying parameters such as the nanoparticle material, the matrix material, the nanoparticle size, and the interparticle distance, the electronic behavior of the nanoparticle arrays can be substantially tuned and controlled. Furthermore, via the excitation of surface plasmon polaritons, the nanoparticles can be optically excited and electronically read-out. The versatility and possible applications of well-ordered nanoparticle arrays has been demonstrated by the realization of switching devices triggered optically or chemically and by the demonstration of chemical and mechanical sensing. Interestingly, hexagonal nanoparticle arrays may also become a useful platform to study the physics of collective plasmon resonances that can be described as Dirac-like bosonic excitations.

• Guiding of Electrons in a Few-Mode Ballistic Graphene Channel
P. Rickhaus, M. -H. Liu, P. Makk, R. Maurand, S. Hess, S. Zihlmann, M. Weiss, K. Richter, and Schönenberger Richter (Uni. C. -. in cooperation with group Regensburg).
Nano Letters, 15(9):5819-5825, sep 2015. arXiv:1509.02653
[Abstract]

In graphene, the extremely fast charge carriers can be controlled by electron-optical elements, such as waveguides, in which the transmissivity is tuned by the wavelength. In this work, charge carriers are guided in a suspended ballistic few-mode graphene channel, defined by electrostatic gating. By depleting the channel, a reduction of mode number and steps in the conductance are observed, until the channel is completely emptied. The measurements are supported by tight-binding transport calculations including the full electrostatics of the sample.

• Formation Mechanism of Metal–Molecule–Metal Junctions: Molecule-Assisted Migration on Metal Defects
D. Thompson, J. Liao, M. Nolan, A. J. Quinn, C. A. Nijhuis, C. O’Dwyer, P. N. Nirmalraj, C. Schönenberger, and M. Calame.
The Journal of Physical Chemistry C, 119(33):19438-19451, aug 2015.
[Abstract]

Activation energies, Ea, measured from molecular exchange experiments are combined with atomic-scale calculations to describe the migration of bare Au atoms and Au–alkanethiolate species on gold nanoparticle surfaces during ligand exchange for the creation of metal–molecule–metal junctions. It is well-known that Au atoms and alkanethiol–Au species can diffuse on gold with sub-1 eV barriers, and surface restructuring is crucial for self-assembled monolayer (SAM) formation for interlinking nanoparticles and in contacting nanoparticles to electrodes. In the present work, computer simulations reveal that naturally occurring ridges and adlayers on Au(111) are etched and resculpted by migration of alkanethiolate–Au species toward high coordination kink sites at surface step edges. The calculated energy barrier, Eb, for diffusion via step edges is 0.4–0.7 eV, close to the experimentally measured Ea of 0.5–0.7 eV. By contrast, putative migration from isolated nine-coordinated terrace sites and complete Au unbinding from the surface incur significantly larger barriers of +1 and +3 eV, respectively. Molecular van der Waals packing energies are calculated to have negligible effect on migration barriers for typically used molecules (length < 2.5 nm), indicating that migration inside SAMs does not change the size of the migration barrier. We use the computational methodology to propose a means of creating Au nanoparticle arrays via selective replacement of citrate protector molecules by thiocyanate linker molecules on surface step sites. This work also outlines the possibility of using Au/Pt alloys as possible candidates for creation of contacts that are well-formed and long-lived because of the superior stability of Pt interfaces against atomic migration.

• Fork stamping of pristine carbon nanotubes onto ferromagnetic contacts for spin-valve devices
J. Gramich, A. Baumgartner, M. Muoth, C. Hierold, and C. Schönenberger.
physica status solidi (b), 252(11):2496-2502, jul 2015. arXiv:1504.05693
[Abstract]

We present a fabrication scheme called ‘fork stamping’ optimized for the dry transfer of individual pristine carbon nanotubes (CNTs) onto ferromagnetic contact electrodes fabricated by standard lithography. We demonstrate the detailed recipes for a residue-free device fabrication and in-situ current annealing on suspended CNT spin-valve devices with ferromagnetic Permalloy (Py) contacts and report preliminary transport characterization and magnetoresistance experiments at cryogenic temperatures. This scheme can directly be used to implement more complex device structures, including multiple gates or superconducting contacts.

• Graphene spintronics: the European Flagship perspective
S. Roche, J. Akerman, B. Beschoten, J. -C. Charlier, M. Chshiev, S. P. Dash, B. Dlubak, J. Fabian, A. Fert, M. Guimaraes, F. Guinea, I. Grigorieva, C. Schönenberger, P. Seneor, C. Stampfer, S. O.Valenzuela, X. Waintal, and B. van Wees.
2D Materials, 2(3):30202, jul 2015.
[Abstract]

We review current challenges and perspectives in graphene spintronics, which is one of the most promising directions of innovation, given its room-temperature long-spin lifetimes and the ability of graphene to be easily interfaced with other classes of materials (ferromagnets, magnetic insulators, semiconductors, oxides, etc), allowing proximity effects to be harvested. The general context of spintronics is first discussed together with open issues and recent advances achieved by the Graphene Spintronics Work Package consortium within the Graphene Flagship project. Based on such progress, which establishes the state of the art, several novel opportunities for spin manipulation such as the generation of pure spin current (through spin Hall effect) and the control of magnetization through the spin torque phenomena appear on the horizon. Practical applications are within reach, but will require the demonstration of wafer-scale graphene device integration, and the realization of functional prototypes employed for determined applications such as magnetic sensors or nano-oscillators. This is a specially commissioned editorial from the Graphene Flagship Work Package on Spintronics. This editorial is part of the 2D Materials focus collection on ‘Progress on the science and applications of two-dimensional materials,’ published in association with the Graphene Flagship. It provides an overview of key recent advances of the spintronics work package as well as the mid-term objectives of the consortium.

• Gigahertz Quantized Charge Pumping in Bottom-Gate-Defined InAs Nanowire Quantum Dots
S. d’Hollosy, M. Jung, A. Baumgartner, V. A. Guzenko, M. H. Madsen, J. Nygård, and C. Schönenberger.
Nano Letters, 15(7):4585-4590, jul 2015. arXiv:1509.01574
[Abstract]

Semiconducting nanowires (NWs) are a versatile, highly tunable material platform at the heart of many new developments in nanoscale and quantum physics. Here, we demonstrate charge pumping, that is, the controlled transport of individual electrons through an InAs NW quantum dot (QD) device at frequencies up to 1.3 GHz. The QD is induced electrostatically in the NW by a series of local bottom gates in a state of the art device geometry. A periodic modulation of a single gate is enough to obtain a dc current proportional to the frequency of the modulation. The dc bias, the modulation amplitude and the gate voltages on the local gates can be used to control the number of charges conveyed per cycle. Charge pumping in InAs NWs is relevant not only in metrology as a current standard, but also opens up the opportunity to investigate a variety of exotic states of matter, for example, Majorana modes, by single electron spectroscopy and correlation experiments.

• Clean carbon nanotubes coupled to superconducting impedance-matching circuits
V. Ranjan, G. Puebla-Hellmann, M. Jung, T. Hasler, A. Nunnenkamp, M. Muoth, C. Hierold, A. Wallraff, and C. Schönenberger.
Nature Communications, 6:7165, may 2015. arXiv:1505.04681
[Abstract]

Coupling carbon nanotube devices to microwave circuits offers a significant increase in bandwidth and signal-to-noise ratio. These facilitate fast non-invasive readouts important for quantum information processing, shot noise and correlation measurements. However, creation of a device that unites a low-disorder nanotube with a low-loss microwave resonator has so far remained a challenge, due to fabrication incompatibility of one with the other. Employing a mechanical transfer method, we successfully couple a nanotube to a gigahertz superconducting matching circuit and thereby retain pristine transport characteristics such as the control over formation of, and coupling strengths between, the quantum dots. Resonance response to changes in conductance and susceptance further enables quantitative parameter extraction. The achieved near matching is a step forward promising high-bandwidth noise correlation measurements on high impedance devices such as quantum dot circuits.

• Sensing with Advanced Computing Technology: Fin Field Effect Transistors with High-K Gate Stack on Bulk Silicon
S. Rigante, P. Scarbolo, M. Wipf, R. L. Stoop, K. Bedner, E. Buitrago, A. Bazigos, D. Bouvet, M. Calame, C. Schönenberger, and A. M. Ionescu..
ACS Nano, 9(5):4872-4881, may 2015.
[Abstract]

{Field-effect transistors (FETs) form an established technology for sensing applications. However, recent advancements and use of high-performance multigate metal–oxide semiconductor FETs (double-gate, FinFET, trigate, gate-all-around) in computing technology, instead of bulk MOSFETs, raise new opportunities and questions about the most suitable device architectures for sensing integrated circuits. In this work, we propose pH and ion sensors exploiting FinFETs fabricated on bulk silicon by a fully CMOS compatible approach, as an alternative to the widely investigated silicon nanowires on silicon-on-insulator substrates. We also provide an analytical insight of the concept of sensitivity for the electronic integration of sensors. N-channel fully depleted FinFETs with critical dimensions on the order of 20 nm and HfO2 as a high-k gate insulator have been developed and characterized, showing excellent electrical properties, subthreshold swing, SS ~ 70 mV/dec, and on-to-off current ratio, Ion/Ioff ~ 10^6, at room temperature. The same FinFET architecture is validated as a highly sensitive, stable, and reproducible pH sensor. An intrinsic sensitivity close to the Nernst limit

• Snake trajectories in ultraclean graphene p–n junctions
P. Rickhaus, P. Makk, Ming-Hao Liu, E. Tovari, M. Weiss, R. Maurand, and C. Schönenberger.
Nature Communications, 6:6470, mar 2015. arXiv:1502.01935
[Abstract]

Snake states are trajectories of charge carriers curving back and forth along an interface. There are two types of snake states, formed by either inverting the magnetic field direction or the charge carrier type at an interface. The former has been demonstrated in GaAs–AlGaAs heterostructures, whereas the latter has become conceivable only with the advance of ballistic graphene where a gap-less p–n interface governed by Klein tunnelling can be formed. Such snake states were hidden in previous experiments due to limited sample quality. Here we report on magneto-conductance oscillations due to snake states in a ballistic suspended graphene p–n junction, which occur already at a very small magnetic field of 20 mT. The visibility of 30 percent is enabled by Klein collimation. Our finding is firmly supported by quantum transport simulations. We demonstrate the high tunability of the device and operate it in different magnetic field regimes.

• Entanglement Detection with Non-Ideal Ferromagnetic Detectors
P. Rozek, P. Busz, W. Klobus, D. Tomaszewski, A. Grudka, A. Baumgartner, C. Schönenberger, and J. Martinek.
Acta Physica Polonica A, 127(2):493-495, feb 2015.
[Abstract]

Entangled states are essential in basics quantum communication protocols and quantum cryptography. Ferromagnetic contacts can work as a spin detector, giving possibility of converting information about electron spin to the electric charge, and therefore, detection of entangled states with the electric current measurements is possible. Method of conrming entanglement with non-ideal detectors is presented, the impact of decoherence and noise on states and quality of entanglement is discussed. Entanglement witness (EW) operator method is compared with the CHSH inequalities approach. Required spin polarization for the EW is lower than for the CHSH inequalities. System with asymmetric spin polarizations of detectors was analyzed, including the CHSH inequalities and the EW method.

• Scalable Tight-Binding Model for Graphene
Ming-Hao Liu, P. Rickhaus, P. Makk, E. Tovari, R. Maurand, F. Tkatschenko, M. Weiss, C. Schönenberger, and K. Richter.
Phys. Rev. Lett., 114(3):36601, jan 2015. arXiv:1407.5620
[Abstract]

Artificial graphene consisting of honeycomb lattices other than the atomic layer of carbon has been shown to exhibit electronic properties similar to real graphene. Here, we reverse the argument to show that transport properties of real graphene can be captured by simulations using “theoretical artificial graphene.” To prove this, we first derive a simple condition, along with its restrictions, to achieve band structure invariance for a scalable graphene lattice. We then present transport measurements for an ultraclean suspended single-layer graphene pn junction device, where ballistic transport features from complex Fabry-Pérot interference (at zero magnetic field) to the quantum Hall effect (at unusually low field) are observed and are well reproduced by transport simulations based on properly scaled single-particle tight-binding models. Our findings indicate that transport simulations for graphene can be efficiently performed with a strongly reduced number of atomic sites, allowing for reliable predictions for electric properties of complex graphene devices. We demonstrate the capability of the model by applying it to predict so-far unexplored gate-defined conductance quantization in single-layer graphene.

### 2014

• Local electrical tuning of the nonlocal signals in a Cooper pair splitter
G. Fülöp, S. d’Hollosy, A. Baumgartner, P. Makk, V. A. Guzenko, M. H. Madsen, J. Nygård, C. Schönenberger, and S. Csonka.
Physical Review B, 90:235412, dec 2014. arXiv:1409.0818
[Abstract]

A Cooper pair splitter consists of a central superconducting contact, S, from which electrons are injected into two parallel, spatially separated quantum dots (QDs). This geometry and electron interactions can lead to correlated electrical currents due to the spatial separation of spin-singlet Cooper pairs from S. We present experiments on such a device with a series of bottom gates, which allows for spatially resolved tuning of the tunnel couplings between the QDs and the electrical contacts and between the QDs. Our main findings are gate-induced transitions between positive conductance correlation in the QDs due to Cooper pair splitting and negative correlations due to QD dynamics. Using a semi-classical rate equation model we show that the experimental findings are consistent with in-situ electrical tuning of the local and nonlocal quantum transport processes. In particular, we illustrate how the competition between Cooper pair splitting and local processes can be optimized in such hybrid nanostructures.

• Random telegraph signals in molecular junctions
J. Brunner, M. T. Gonzalez, C. Schönenberger, and M. Calame.
J. Phys.: Condens. Matter, 26:474202, oct 2014.
[Abstract]

We investigate conductance fluctuations in molecular junctions using a mechanically controllable break junction setup in a liquid environment. In contrast to conventional break junction measurements, time-dependent conductance signals were recorded while reducing the gap size between the two contact electrodes. Only small amplitude fluctuations of the conductance are observed when measuring in pure solvent. Conductance traces recorded in solutions containing alkanedithiols show significantly larger fluctuations which can take the form of random telegraph signals. Such signals emerge in a limited conductance range, which corresponds well to the known molecular conductance of the compounds investigated. These large-amplitude fluctuations are attributed to the formation and thermally driven breaking of bonds between a molecule and a metal electrode and provide a still poorly explored source of information on the dynamics of molecular junctions formation. The lifetimes of the high and low conductance states are found to vary between 0.1 ms and 0.1 s

• Large-scale fabrication of BN tunnel barriers for graphene spintronics
W. Fu, P. Makk, R. Maurand, M. Bräuninger, and C. Schönenberger.
Journal of Applied Physics, 116(20):74306, aug 2014. arXiv:1407.1439
[Abstract]

We have fabricated graphene spin-valve devices utilizing scalable materials made from chemical vapor deposition (CVD). Both the spin-transporting graphene and the tunnel barrier material are CVD-grown. The tunnel barrier is realized by Hexagonal boron nitride, used either as a monolayer or bilayer and placed over the graphene. Spin transport experiments were performed using ferromagnetic contacts deposited onto the barrier. We find that spin injection is still greatly suppressed in devices with a monolayer tunneling barrier due to resistance mismatch. This is, however, not the case for devices with bilayer barriers. For those devices, a spin relaxation time of ~260 ps intrinsic to the CVD graphene material is deduced. This time scale is comparable to those reported for exfoliated graphene, suggesting that this CVD approach is promising for spintronic applications which require scalable materials.

• Sensor system including silicon nanowire ion sensitive FET arrays and CMOS readout
P. Livi, A. Shadmani, M. Wipf, R. L. Stoop, J. Rothe, Y. Chen, M. Calame, and C. Schönenberger.
Sensors and Actuators B, 204:568-577, aug 2014.
[Abstract]

We present a highly sensitive chemical sensor system including a chip with an array of silicon nanowire ISFETs and a CMOS chip with custom-designed signal-conditioning circuitry. The CMOS circuitry, comprising 8 sigma–delta (Σ–Δ) modulators and 8 current-to-frequency converters, has been interfaced to each of the nanowires to apply a constant voltage for measuring the respective current through the nanowire. Each nanowire has a dedicated readout channel, so that no multiplexing is required, which helps to avoid leakage current issues. The analog signal has been digitized on chip and transmitted to a host PC via a FPGA. The system has been successfully fabricated and tested and features, depending on the settings, noise values as low as 8.2 pARMS and a resolution of 13.3 bits while covering an input current range from 200 pA to 3 μA. The two readout architectures (Σ–Δ and current to frequency) have been compared, and measurements showing the advantages of combining a CMOS readout with silicon nanowire sensors are presented: (1) simultaneous readout of different silicon nanowires for high-temporal-resolution experiments and parallel sensor experiments (results from pH and KCl concentration sweeps are presented); (2) high speed measurements showing how the CMOS chip can enhance the performance of the nanowire sensors by compensating its non-idealities as a consequence of hysteresis.

• Fabrication of ballistic suspended graphene with local-gating
R. Maurand, P. Rickhaus, P. Makk, S. Hess, E. Tovari, C. Handschin, M. Weiss, and C. Schönenberger.
Carbon, 79:486-492, aug 2014. arXiv:1409.4751
[Abstract]

Herein we discuss the fabrication of ballistic suspended graphene nanostructures supplemented with local gating. Using in situ current annealing, we show that exceptional high mobilities can be obtained in these devices. A detailed description is given of the fabrication of bottom and different top-gate structures, which enable the realization of complex graphene structures. We have studied the basic building block, the p-n junction in detail, where a striking oscillating pattern was observed, which can be traced back to Fabry–Perot oscillations that are localized in the electronic cavities formed by the local gates. Finally we show some examples how the method can be extended to incorporate multi-terminal junctions or shaped graphene. The structures discussed here enable the access to electron-optics experiments in ballistic graphene.

• CVD graphene for electrical quantum metrology
K. Thodkar, C. Nef, W. Fu, C. Schönenberger, M. Calame, F. Lüönd, F. Overney, B. Jeckelmann, and B. Jeanneret.
IEEE Proceedings of the Conference on Precision Electromagnetic Measurements (CPEM 2014), pages 540-541, aug 2014.
[Abstract]

Graphene, a two dimensional material with sp2 hybridized carbon atoms arranged in honey comb lattice, is known for its unique electronic and mechanical properties. Soon after the isolation of 2D graphene crystals Quantum Hall effect (QHE) has been observed in this material at room temperature. The Quantum Hall plateaus in graphene have large spacing between the Landau levels in comparison to other 2DEGs, which makes it an ideal material for a quantum resistance standard defined by the electron charge and Planck s constant. We will present results for graphene by Chemical Vapor Deposition (CVD) and transferred to SiO2/Si using different techniques. The transferred graphene films were patterned into millimeter scale Hall bar geometry and characterized using confocal Raman spectroscopy. First electrical transport measurements will be presented.

• Carbon nanotube quantum dots on hexagonal boron nitride
A. Baumgartner, G. Abulizi, K. Watanabe, T. Taniguchi, J. Gramich, and C. Schönenberger.
Appl. Phys. Lett., 105:23111, jun 2014. arXiv:1406.0897
[Abstract]

We report the fabrication details and low-temperature characteristics of the first carbon nanotube (CNT) quantum dots on flakes of hexagonal boron nitride (hBN) as substrate. We demonstrate that CNTs can be grown on hBN by standard chemical vapor deposition and that standard scanning electron microscopy imaging and lithography can be employed to fabricate nanoelectronic structures when using optimized parameters. This proof of concept paves the way to more complex devices on hBN, with more predictable and reproducible characteristics and electronic stability.

• Regulating a Benzodifuran Single Molecule Redox Switch via Electrochemical Gating and Optimization of Molecule/Electrode Coupling
Zhihai Li, Hui Li, Songjie Chen, Toni Fröhlich, Chenyi Yi, Christian Schönenberger, Michel Calame, Silvio Decurtins, Shi-Xia Liu, and Eric Borguet.
J. Am. Chem. Soc., 136:8867-8870, jun 2014.
[Abstract]

We report a novel strategy for the regulation of charge transport through single molecule junctions via the combination of external stimuli of electrode potential, internal modulation of molecular structures, and optimization of anchoring groups. We have designed redox-active benzodifuran (BDF) compounds as functional electronic units to fabricate metal–molecule–metal (m–M–m) junction devices by scanning tunneling microscopy (STM) and mechanically controllable break junctions (MCBJ). The conductance of thiol-terminated BDF can be tuned by changing the electrode potentials showing clearly an off/on/off single molecule redox switching effect. To optimize the response, a BDF molecule tailored with carbodithioate (−CS2–) anchoring groups was synthesized. Our studies show that replacement of thiol by carbodithioate not only enhances the junction conductance but also substantially improves the switching effect by enhancing the on/off ratio from 2.5 to 8.

• High-yield fabrication of nm-size gaps in monolayer CVD graphene
C. Nef, L. Pósa, P. Makk, W. Fu, A. Halbritter, C. Schönenberger, and M. Calame.
Nanoscale, 6:7249-7254, may 2014.
[Abstract]

Herein we demonstrate the controlled and reproducible fabrication of sub-5 nm wide gaps in single-layer graphene electrodes. The process is implemented for graphene grown via chemical vapor deposition using an electroburning process at room temperature and in vacuum. A yield of over 95 percent for the gap formation is obtained. This approach allows producing single-layer graphene electrodes for molecular electronics at a large scale. Additionally, from Raman spectroscopy and electroburning carried out simultaneously, we can follow the heating process and infer the temperature at which the gap formation happens.

• Optimized fabrication and characterization of carbon nanotube spin valves
J. Samm, J. Gramich, A. Baumgartner, M. Weiss, and C. Schönenberger.
J. Appl. Phys., 115:174309, may 2014. arXiv:1312.0159
[Abstract]

We report an improved fabrication scheme for carbon based nanospintronic devices and demonstrate the necessity for a careful data analysis to investigate the fundamental physical mechanisms leading to magnetoresistance. The processing with a low-density polymer and an optimised recipe allows us to improve the electrical, magnetic, and structural quality of ferromagnetic Permalloy contacts on lateral carbon nanotube (CNT) quantum dot spin valve devices, with comparable results for thermal and sputter deposition of the material. We show that spintronic nanostructures require an extended data analysis, since the magnetization can affect all characteristic parameters of the conductance features and lead to seemingly anomalous spin transport. In addition, we report measurements on CNT quantum dot spin valves that seem not to be compatible with the orthodox theories for spin transport in such structures.

• Rendering graphene supports hydrophilic with non-covalent aromatic functionalization for transmission electron microscopy
R. S. Pantelic, W. Fu, C. Schönenberger, and H. Stahlberg.
Appl. Phys. Lett., 104:134103, apr 2014.
[Abstract]

Amorphous carbon films have been routinely used to enhance the preparation of frozen-hydrated transmission electron microscopy (TEM) samples, either in retaining protein concentration, providing mechanical stability or dissipating sample charge. However, strong background signal from the amorphous carbon support obstructs that of the sample, and the insulating properties of amorphous carbon films preclude any efficiency in dispersing charge. Graphene addresses the limitations of amorphous carbon. Graphene is a crystalline material with virtually no phase or amplitude contrast and unparalleled, high electrical carrier mobility. However, the hydrophobic properties of graphene have prevented its routine application in Cryo-TEM. This letter reports a method for rendering graphene TEM supports hydrophilic – a convenient approach maintaining graphene’s structural and electrical properties based on non-covalent, aromatic functionalization.

• Entanglement witnessing and quantum cryptography with nonideal ferromagnetic detectors
W. Kobus, A. Grudka, A. Baumgartner, D. Tomaszewski, C. Schönenberger, and Jan Martinek.
Phys. Rev. B, 89:125404, mar 2014. arXiv:1310.5640
[Abstract]

We investigate theoretically the use of non-ideal ferromagnetic contacts as a mean to detect quantum entanglement of electron spins in transport experiments. We use a designated entanglement witness and find a minimal spin polarization of η>1/3–√≈58 required to demonstrate spin entanglement. This is significantly less stringent than the ubiquitous tests of Bell’s inequality with η>1/2–√4≈84. In addition, we discuss the impact of decoherence and noise on entanglement detection and apply the presented framework to a simple quantum cryptography protocol. Our results are directly applicable to a large variety of experiments.

• Investigation of the dominant 1/f Noise Source in Silicon Nanowire Sensors
K. Bedner, V. A. Guzenko, A. Tarasov, M. Wipf, L. Stoop, S. Rigante, J. Brunner, W. Fu, C. David, M. Calame, J. Gobrecht, and C. Schönenberger.
Sensor & Actuators B, 191:270-275, feb 2014.
[Abstract]

We analyzed 1/f noise in silicon nanowire ion-sensitive field-effect transistors (SiNW-ISFETs) having different wire widths ranging from 100 nm to 1 μm and operated under different gating conditions in order to determine the noise source and the sensor accuracy. We find that the gate-referred voltage noise SVG (power spectral density) is constant over a large range of SiNWs resistances tuned by a DC gate voltage. The measurements of SVG for SiNWs with two different gate-oxide thicknesses, but otherwise similar device parameters, are only compatible with the so-called trap state noise model in which the source of 1/f noise is due to trap states residing in the gate oxide (most likely in the interface between the semiconductor and the oxide). SVG is found to be inversely proportional to the wire width for constant wire length. From the noise data we determine a sensor accuracy of 0.017\% of a full Nernstian shift of 60 mV/pH for a SiNW wire with a width of 1 μm. No influence of the ions in the buffer solution was found

• Electrolyte gate dependent high-frequency measurement of graphene field-effect transistor for sensing applications
W. Fu, El M. Abbassi, T. Hasler, M. Jung, M. Steinacher, M. Calame, C. Schönenberger, G. Puebla-Hellmann, S. Hellmüller, T. Ihn, and A. Wallraff.
Appl. Phys. Lett., 104:13102, jan 2014. arXiv:1401.0381
[Abstract]

We performed radiofrequency (RF) reflectometry measurements at 2�4 GHz on electrolyte-gated graphene field-effect transistors, utilizing a tunable stub-matching circuit for impedance matching. We demonstrate that the gate voltage dependent RF resistivity of graphene can be deduced, even in the presence of the electrolyte which is in direct contact with the graphene layer. The RF resistivity is found to be consistent with its DC counterpart in the full gate voltage range. Furthermore, in order to access the potential of high-frequency sensing for applications, we demonstrate time-dependent gating in solution with nanosecond time resolution.

• Nonlocal spectroscopy of Andreev bound states
J. Schindele, A. Baumgartner, R. Maurand, M. Weiss, and C. Schönenberger.
Phys. Rev. B, 89:45422, jan 2014. arXiv:1311.0659
[Abstract]

We experimentally investigate Andreev bound states (ABSs) in a carbon nanotube quantum dot (QD) connected to a superconducting Nb lead (S). A weakly coupled normal metal contact acts as a tunnel probe that measures the energy dispersion of the ABSs. Moreover, we study the response of the ABS to nonlocal transport processes, namely, Cooper pair splitting and elastic co-tunnelling, which are enabled by a second QD fabricated on the same nanotube on the opposite side of S. We find an appreciable nonlocal conductance with a rich structure, including a sign reversal at the ground-state transition from the ABS singlet to a degenerate magnetic doublet. We describe our device by a simple rate equation model that captures the key features of our observations and demonstrates that the sign of the nonlocal conductance is a measure for the charge distribution of the ABS, given by the respective Bogoliubov-de Gennes amplitudes u and v.

### 2013

• g-factor anisotropy in nanowire-based InAs quantum dots
S. d’Hollosy, G. Fabian, A. Baumgartner, J. Nygard, and C. and Schönenberger.
AIP Conference Proceedings, 1566:359, dec 2013. arXiv:1309.0726
[Abstract]

The determination and control of the electron g-factor in semiconductor quantum dots (QDs) are fundamental prerequisites in modern concepts of spintronics and spin-based quantum computation. We study the dependence of the g-factor on the orientation of an external magnetic field in quantum dots (QDs) formed between two metallic contacts on stacking fault free InAs nanowires. We extract the g-factor from the splitting of Kondo resonances and find that it varies continuously in the range between |g| = 5 and 15.

• Ballistic interferences in suspended graphene
P. Rickhaus, R. Maurand, M. Weiss, C. Schönenberger, Ming-Hao Liu, and K. Richter.
Nature Communications, 4(2342):1-6, aug 2013. arXiv:1304.6590
[Abstract]

Graphene is the 2-dimensional (2D) carbon allotrope with the atoms arranged in a honeycomb lattice [1]. The low-energy electronic excitations in this 2D crystal are described by massless Dirac fermions that have a linear dispersion relation similar to photons [2, 3]. Taking advantage of this optics-like electron dynamics, generic optical elements like lenses, beam splitters and wave guides have been proposed for electrons in engineered and ballistic graphene [4, 5]. Tuning of these elements rely on the ability to adjust the carrier concentration in defined areas, including the possibility to create bipolar regions of opposite charge (p-n regions). However, the combination of ballistic transport and complex electrostatic gating remain challenging. Here, we report on the fabrication and characterization of fully suspended graphene p-n junctions. By local electrostatic gating, resonant cavities can be defined, leading to complex Fabry-Perot interference patterns in the unipolar and the bipolar regime. The amplitude of the observed conductance oscillations account for quantum interference of electrons that propagate ballistically over long distances exceeding 1 micrometer. We also demonstrate that the visibility of the interference pattern is enhanced by Klein collimation at the p-n interface [6, 7]. This finding paves the way to more complex gate controlled ballistic graphene devices and brings electron optics in graphene closer to reality.

• Viewpoint: To Screen or Not to Screen, That is the Question
R. Maurand and C. Schönenberger.
APS Physics Viewpoints, 6:75, jul 2013.

• Low-bias active control of TeraHertz-waves by coupling large-area CVD-graphene to a TeraHertz-Metamaterial
F. Valmorra, G. Scalari, C. Maissen, W. Fu, C. Schönenberger, J. W. Choi, H. G. Park, Hyung Gyu, M. Beck, and J. Faist.
Nano Lett., 13(7):3193-3198, jul 2013.
[Abstract]

We propose an hybrid graphene/metamaterial device based on terahertz electronic split-ring resonators directly evaporated on top of a large-area single-layer CVD graphene. Room temperature time-domain spectroscopy measurements in the frequency range from 250 GHz to 2.75 THz show that the presence of the graphene strongly changes the THz metamaterial transmittance on the whole frequency range. The graphene gating allows active control of such interaction, showing a modulation depth of 11.5% with an applied bias of 10.6 V. Analytical modeling of the device provides a very good qualitative and quantitative agreement with the measured device behavior. The presented system shows potential as a THz modulator and can be relevant for strong light–matter coupling experiments.

• pH Response of Silicon Nanowire Sensors: Impact of Nanowire Width and Gate Oxide
K. Bedner, V. A. Guzenko, A. Tarasov, M. Wipf, L. Stoop, D. Just, S. Rigante, W. Fu, R. A. Minamisawa, C. David, M. Calame, J. Gobrecht, and C. Schönenberger.
Sensors and Materials, 25(8):567, jun 2013.
[Abstract]

We present a systematic study of the performance of silicon nanowires (SiNWs) with different widths when they are used as ion-sensitive field-effect transistors (ISFETs) in pH-sensing experiments. The SiNW widths ranged from 100 nm to 1 mu m. The SiNW-ISFETs were successfully fabricated from silicon-on-insulator (SOT) wafers with Al2O3 or HfO2 as gate dielectric. All the SiNWs showed a pH Response close to the Nernstian limit of 59.5 mV/pH at 300 K, independent of their width, or the investigated gate dielectric or operating mode. Even nanowires (NWs) in the 100 nm range operated reliably without degradation of their functionality. This result is of importance for a broad research field using SiNW sensors as a candidate for future applications.

• Entanglement witnessing in superconducting beamsplitters
H. Soller, L. Hofstetter, and D. Reeb.
EPL (Europhysics Letters), 102(5):50009, jun 2013. arXiv:1305.7061v1
[Abstract]

We analyse a large class of superconducting beamsplitters for which the Bell parameter (CHSH violation) is a simple function of the spin detector efficiency. For these superconducting beamsplitters all necessary information to compute the Bell parameter can be obtained in Y-junction setups for the beamsplitter. Using the Bell parameter as an entanglement witness, we propose an experiment which allows to verify the presence of entanglement in Cooper pair splitters.

• Silicon Nanowire Ion-Sensitive Field-Effect Transistor Array Integrated with a CMOS-based Readout Chip
P. Livi, M. Wipf, A. Tarasov, R. Stoop, K. Bedner, J. Rothe, Y. Chen, A. Stettler, C. Schönenberger, and A. Hierlemann.
Proc. Of IEEE Transducers, Barcelona, SPAIN, 16-20 June 2013, pages 1751-1754, jun 2013.
[Abstract]

We present a biosensor system comprising a chip with an array of silicon nanowire ISFETs and a CMOS chip with the signal-conditioning circuitry. An array of 48 silicon nanowires has been patterned via e-beam lithography. The on-chip circuitry is used to monitor the resistance of the nanowires by applying a constant voltage and measuring the respective current. The analog signal is then digitized on chip and transmitted. The system has been successfully fabricated and tested. Measurements showing the advantage of using CMOS readout are presented: 1) simultaneous readout of different silicon nanowires, 2) low noise and 3) fast measurements.

• pH Response of Silicon Nano\-wire Sensors: Impact of Nano\-wire Width and Gate Oxide
K. Bedner, V. A. Guzenko, A. Tarasov, M. Wipf, R. L. Stoop, D. Just, S. Rigante, W. Fu, R. A. Minamisawa, C. David, M. Calame, J. Gobrecht, and C. Schönenberger.
Sensors and Materials, 25(8):567-576, may 2013.
[Abstract]

We present a systematic study of the performance of silicon nanowires (SiNWs) with different widths when they are used as ion-sensitive field-effect transistors (ISFETs) in pH-sensing experiments. The SiNW widths ranged from 100 nm to 1 micro m. The SiNW-ISFETs were successfully fabricated from silicon-on-insulator (SOI) wafers with Al2O3 or HfO2 as gate dielectric. All the SiNWs showed a pH Response close to the Nernstian limit of 59.5 mV/pH at 300 K, independent of their width, or the investigated gate dielectric or operating mode. Even nanowires (NWs) in the 100 nm range operated reliably without degradation of their functionality. This result is of importance for a broad research field using SiNW sensors as a candidate for future applications.

• Spin symmetry of the bilayer graphene ground state
F. Freitag, M. Weiss, R. Maurand, J. Trbovic, and C. Schönenberger.
Phys. Rev. B, 87(16):161402, apr 2013.
[Abstract]

We show nonlinear transport experiments on clean, suspended bilayer graphene that reveal a gap in the density of states. Looking at the evolution of the gap in magnetic fields of different orientation, we find that the groundstate is a spin-ordered phase. Of the three possible gapped groundstates that are predicted by theory for equal charge distribution between the layers, we can therefore exclude the quantum anomalous Hall phase, leaving the layer antiferromagnet and the quantum spin Hall phase as the only possible gapped groundstates for bilayer graphene

• A Verilog-A model for silicon nanowire biosensors: From theory to verification
P. Livi, K. Bedner, A. Tarasov, Mathias Wipf, Y. Chen, C. Schönenberger, and A. Hierlemann.
Sensors and Actuators B, 179(doi:10.1016/j.snb.2012.09.026):293-300, mar 2013.
[Abstract]

Silicon nanowires offer great potential as highly sensitive biosensors. Since the signals they produce are quite weak and noisy, the use of integrated circuits is preferable to read out and digitize these signals as quickly as possible following the sensing event to take full advantage of the properties of the nanowires. In order to design optimized and tailored circuits, simulations involving the sensor itself in the design phase are needed. We propose here a Verilog-A model for silicon nanowire-based biosensors. The model can easily be applied using commercially available Electronic Design Automation (EDA) tools that are commonly used for integrated circuit design and simulations. The model is quite general and comprehensive; it can be used to simulate different types of sensing events, while still being quite simple and undemanding in terms of computational power. The model is described in detail and verified with measurements from two different nanowire sensors featuring aluminum-oxide and hafnium-oxide coatings. Good agreement has been achieved in all cases, with errors never exceeding 21 percent. The complete Verilog-A code is made available in the Appendix.

• Hydrogen plasma microlithography of graphene supported on a Si/SiO2 substrate
B. Eren, T. Glatzel, M. Kisiel, W. Fu, R. Pawlak, U. Gysin, C. Nef, L. Marot, M. Calame, C. Schönenberger, and E. Meyer.
Appl. Phys. Lett., 102:71602, feb 2013.
[Abstract]

In this work, a silicon stencil mask with a periodic pattern is used for hydrogen plasma microlithography of single layer graphene supported on a Si/SiO2 substrate. Obtained patterns are imaged with Raman microscopy and Kelvin probe force microscopy, thanks to the changes in the vibrational modes and the contact potential difference (CPD) of graphene after treatment. A decrease of 60 meV in CPD as well as a significant change of the D/G ratio in the Raman spectra can be associated with a local hydrogenation of graphene, while the topography remains invariant to the plasma exposure.

• High mobility graphene ion-sensitive field-effect transistors by noncovalent functionalization
W. Fu, C. Nef, A. Tarasov, M. Wipf, R. Stoop, O. Knopfmacher, M. Weiss, M. Calame, and C. Schönenberger.
Nanoscale, 5:12104, 2013.
[Abstract]

Noncovalent functionalization is a well-known nondestructive process for property engineering of carbon nanostructures, including carbon nanotubes and graphene. However, it is not clear to what extend the extraordinary electrical properties of these carbon materials can be preserved during the process. Here, we demonstrated that noncovalent functionalization can indeed delivery graphene field-effect transistors (FET) with fully preserved mobility. In addition, these high-mobility graphene transistors can serve as a promising platform for biochemical sensing applications.

• Ultraclean Single, Double, and Triple Carbon Nanotube Quantum Dots with Recessed Re Bottom Gates
M. Jung, J. Schindele, S. Nau, M. Weiss, A. Baumgartner, and C. Schönenberger.
Nano Lett., 13:4522-4526, 2013.
[Abstract]

We demonstrate that ultraclean single, double, and triple quantum dots (QDs) can be formed reliably in a carbon nanotube (CNT) by a straightforward fabrication technique. The QDs are electrostatically defined in the CNT by closely spaced metallic bottom gates deposited in trenches in SiO2 by sputter deposition of Re. The carbon nanotubes are then grown by chemical vapor deposition (CVD) across the trenches and contacted using conventional resist-based electron beam lithography. Unlike in previous work, the devices exhibit reproducibly the characteristics of ultraclean QDs behavior even after the subsequent electron beam lithography and chemical processing steps. We specifically demonstrate the high quality using CNT devices with two narrow bottom gates and one global back gate. Tunable by the gate voltages, the device can be operated in four different regimes: (i) fully p-type with ballistic transport between the outermost contacts (over a length of 700 nm), (ii) clean n-type single QD behavior where a QD can be induced by either the left or the right bottom gate, (iii) n-type double QD, and (iv) triple bipolar QD where the middle QD has opposite doping (p-type). Our simple fabrication scheme opens up a route to more complex devices based on ultraclean CNTs, since it allows for postgrowth processing.

• Two Indistinguishable Electrons Interfere in an Electronic Device
C. Schönenberger.
Science, 339:1041-1042, 2013.

• Selective Sodium Sensing with Gold-Coated Silicon Nanowire Field-Effect Transistors in a Differential Setup
M. Wipf, R. L. Stoop, A. Tarasov, K. Bedner, W. Fu, I. A. Wright, C. J. Martin, E. C. Constable, M. Calame, and C. Schönenberger.
ACS Nano, 7(7):5978-5983, 2013.
[Abstract]

Ion-sensitive field-effect transistors based on silicon nanowires with high dielectric constant gate oxide layers (e.g., Al2O3 or HfO2) display hydroxyl groups which are known to be sensitive to pH variations but also to other ions present in the electrolyte at high concentration. This intrinsically nonselective sensitivity of the oxide surface greatly complicates the selective sensing of ionic species other than protons. Here, we modify individual nanowires with thin gold films as a novel approach to surface functionalization for the detection of specific analytes. We demonstrate sodium ion (Na+) sensing by a self-assembled monolayer (SAM) of thiol-modified crown ethers in a differential measurement setup. A selective Na+ response of ≈−44 mV per decade in a NaCl solution is achieved and tested in the presence of protons (H+), potassium (K+), and chloride (Cl–) ions, by measuring the difference between a nanowire with a gold surface functionalized by the SAM (active) and a nanowire with a bare gold surface (control). We find that the functional SAM does not affect the unspecific response of gold to pH and background ionic species. This represents a clear advantage of gold compared to oxide surfaces and makes it an ideal candidate for differential measurements.

### 2012

• Homogeneity of bilayer graphene
F. Freitag, M. Weiss, R. Maurand, J. Trbovic, and C. Schönenberger.
Solid State Communications, 152(22):2053-2057, nov 2012. arXiv:1207.4424v2
[Abstract]

We present non-linear transport measurements on suspended, current annealed bilayer graphene devices. Using a multi-terminal geometry we demonstrate that devices tend to be inhomogeneous and host two different electronic phases next to each other. Both of these phases show gap-like features of different magnitude in non-linear transport at low charge carrier densities, as already observed in previous studies. Here, we investigate the magnetic field dependence and find that both features grow with increasing field, the smaller one with 0.6meV/T, the larger one with a 5�10 times higher field dependence. We attribute the larger of the two gaps to an interaction induced broken symmetry state and the smaller one to localization in the more disordered parts of the device.

• Near-Unity Cooper Pair Splitting Efficiency
J. Schindele, A. Baumgartner, and C. Schönenberger.
Phys. Rev. Lett., 109(15):157002, oct 2012. arXiv:1204.5777
[Abstract]

The two electrons of a Cooper pair in a conventional superconductor form a spin singlet and therefore a maximally entangled state. Recently, it was demonstrated that the two particles can be extracted from the superconductor into two spatially separated contacts via two quantum dots in a process called Cooper pair plitting (CPS). Competing transport processes, however, limit the efficiency of this process. Here we demonstrate efficiencies up to 90 percent, significantly larger than required to demonstrate interactiondominated CPS, and on the right order to test Bell�s inequality with electrons. We compare the CPS currents through both quantum dots, for which large apparent discrepancies are possible. The latter we explain intuitively and in a semiclassical master equation model. Large efficiencies are required to detect electron entanglement and for prospective electronics-based quantum information technologies.

• Understanding the Electrolyte Background for Biochemical Sensing with Ion-Sensitive Field-Effect Transistors
A. Tarasov, M. Wipf, R. L. Stoop, K. Bedner, W. Fu, V. A. Guzenko, O. Knopfmacher, M. Calame, and C. Schönenberger.
ACS Nano, 6(10):9291-9298, oct 2012.
[Abstract]

Silicon nanowire field-effect transistors have attracted substantial interest for various biochemical sensing applications, yet there remains uncertainty concerning their response to changes in the supporting electrolyte concentration. In this study, we use silicon nanowires coated with highly pH-sensitive hafnium oxide (HfO2) and aluminum oxide (Al2O3) to determine their response to variations in KCl concentration at several constant pH values. We observe a nonlinear sensor response as a function of ionic strength, which is independent of the pH value. Our results suggest that the signal is caused by the adsorption of anions (Cl-) rather than cations (K+) on both oxide surfaces. By comparing the data to three well established models, we have found that none of those can explain the present data set. Finally, we propose a new model which gives excellent quantitative agreement with the data.

• Molecular electronics: functions and features arising from tailor-made molecules
M. Mayor, M. Calame, and R. Waser.
in Nanoelectronics and Information Technology, 3rd ed., Wiley-VCH, 8(3):355-377, sep 2012.

• Sensing with liquid-gated graphene field-effect transistors
W. Fu, C. Nef, A. Tarasov, M. Wipf, R. Stoop, O. Knopfmacher, M. Weiss, M. Calame, and C. Schönenberger.
Proceedings of the IEEE conference on nanotechnology (IEEE-NANO), aug 2012.
[Abstract]

Liquid-gated graphene field-effect transistors (GFETs) with reliable performance are developed. It is revealed that ideal defect-free graphene should be inert to electrolyte composition changes in solution, whereas a defective one responses to electrolyte composition. This finding sheds light on the large variety of pH or ion-induced gate shifts that have been published for GFETs in the recent literature. As a next step to target graphene-based (bio-) chemical sensing platform, non-covalent functionalization of graphene has to be introduced.

• Force-conductance correlation in individual molecular junctions
C. Nef, P. L. T. M. Frederix, J. Brunner, M. Calame, and Schönenberger.
Nanotechnology, 23:365201, aug 2012.
[Abstract]

Conducting atomic force microscopy is an attractive approach enabling the correlation of mechanical and electrical properties in individual molecular junctions. Here we report on measurements of gold-gold and gold-octanedithiol-gold junctions. We introduce two-dimensional histograms in the form of scatter plots to better analyze the correlation between force and conductance. In this representation, the junction-forming octanedithiol compounds lead to a very clear step in the force-conductance data, which is not observed for control monothiol compounds. The conductance found for octanedithiols is in agreement with the idea that junction conductance is dominated by a single molecule.

• True Reference Nanosensor Realized with Silicon Nanowires
A. Tarasov, M. Wipf, K. Bedner, J. Kurz, W. Fu, V. A. Guzenko, O. Knopfmacher, L. Stoop, M. Calame, and C. Schönenberger.
Langmuir, 28(25):9899-9905, jun 2012.
[Abstract]

Conventional gate oxide layers (e.g., SiO2, Al2O3, or HfO2) in silicon field-effect transistors (FETs) provide highly active surfaces, which can be exploited for electronic pH sensing. Recently, great progress has been achieved in pH sensing using compact integrateable nanowire FETs. However, it has turned out to be much harder to realize a true reference electrode, which — while sensing the electrostatic potential — does not respond to the proton concentration. In this work, we demonstrate a highly effective reference sensor, a so-called reference FET, whose proton sensitivity is suppressed by as much as 2 orders of magnitude. To do so, the Al2O3 surface of a nanowire FET was passivated with a self-assembled monolayer of silanes with a long alkyl chain. We have found that a full passivation can be achieved only after an extended period of self-assembling lasting several days at 80 degC. We use this slow process to measure the number of active proton binding sites as a function of time by a quantitative comparison of the measured nonlinear pH-sensitivities to a theoretical model (site-binding model). Furthermore, we have found that a partially passivated surface can sense small changes in the number of active binding sites reaching a detection limit of delta Ns approx 170 1/micron^2 Hz^1/2 at 10 Hz and pH 3.

• Negative Differential Photoconductance in Gold Nanoparticle Arrays in the Coulomb Blockade Regime
M. A. Mangold, M. Calame, M. Mayor, and A. W. Holleitner.
ACS Nano, 6(5):4181-4189, may 2012.
[Abstract]

We investigate the photoconductance of gold nanoparticle arrays in the Coulomb blockade regime. Two-dimensional, hexagonal crystals of nanoparticles are produced by self-assembly. The nanoparticles are weakly coupled to their neighbors by a tunneling conductance. At low temperatures, the single electron charging energy of the nanoparticles dominates the conductance properties of the array. The Coulomb blockade of the nanoparticles can be lifted by optical excitation with a laser beam. The optical excitation leads to a localized heating of the arrays, which in turn gives rise to a local change in conductance and a redistribution of the overall electrical potential in the arrays. We introduce a dual-beam optical excitation technique to probe the distribution of the electrical potential in the nanoparticle array. A negative differential photoconductance is the direct consequence of the redistribution of the electrical potential upon lifting of the Coulomb blockade. On the basis of our model, we calculate the optically induced current from the dark current–voltage characteristics of the nanoparticle array. The calculations closely reproduce the experimental observations.

• Kondo effect and spin-active scattering in ferromagnet-superconductor junctions
H. Soller, L. Hofstetter, S. Csonka, Levy A. Yeyati, C. Schönenberger, and A. Komnik.
Phys. Rev. B, 85(17):174512, may 2012. arXiv:1204.6581v1
[Abstract]

We study the interplay of superconducting and ferromagnetic correlations on charge transport in different geometries with a focus on both a quantum point contact as well as a quantum dot in the even and the odd state with and without spin-active scattering at the interface. In order to obtain a complete picture of the charge transport we calculate the full counting statistics in all cases and compare the results with experimental data. We show that spin-active scattering is an essential ingredient in the description of quantum point contacts. This holds also for quantum dots in an even charge state whereas it is strongly suppressed in a typical Kondo situation. We explain this feature by the strong asymmetry of the hybridisations with the quantum dot and show how Kondo peak splitting in a magnetic field can be used for spin filtering. For the quantum dot in the even state spin-active scattering allows for an explanation of the experimentally observed mini-gap feature.

• Quantum Hall Effect in Graphene with Superconducting Electrodes
P. Rickhaus, M. Weiss, L. Marot, and C. Schönenberger.
Nano Letters, 12(4):1942-1945, apr 2012. arXiv:1303.3394v1
[Abstract]

We have realized an integer quantum Hall system with superconducting contacts by connecting graphene to niobium electrodes. Below their upper critical field of 4 T, an integer quantum Hall effect coexists with superconductivity in the leads but with a plateau conductance that is larger than in the normal state. We ascribe this enhanced quantum Hall plateau conductance to Andreev processes at the graphene–superconductor interface leading to the formation of so-called Andreev edge-states. The enhancement depends strongly on the filling-factor and is less pronounced on the first plateau due to the special nature of the zero energy Landau level in monolayer graphene.

• Spontaneously Gapped Ground State in Suspended Bilayer Graphene
F. Freitag, J. Trbovic, M. Weiss, and C. Schönenberger.
Phys. Rev. Lett., 108(7):76602, feb 2012. arXiv:1104.3816
[Abstract]

Bilayer graphene bears an eight-fold degeneracy due to spin, valley and layer symmetry, allowing for a wealth of broken symmetry states induced by magnetic or electric fields, by strain, or even spontaneously by interaction. We study the electrical transport in clean current annealed suspended bilayer graphene. We find two kind of devices. In bilayers of type B1 the eight-fold zero-energy Landau level (LL) is partially lifted above a threshold field revealing an insulating nu=0 quantum Hall state at the charge neutrality point (CNP). In bilayers of type B2 the LL lifting is full and a gap appears in the differential conductance even at zero magnetic field, suggesting an insulating spontaneously broken symmetry state. Unlike B1, the minimum conductance in B2 is not exponentially suppressed, but remains finite with a value G < e^2/h even in a large magnetic field. We suggest that this phase of B2 is insulating in the bulk and bound by compressible edge states.

• Silicon-Based Ion-Sensitive Field-Effect Transistor Shows Negligible Dependence on Salt Concentration at Constant pH
O. Knopfmacher, A. Tarasov, M. Wipf, W. Fu, M. Calame, and C. Schönenberger.
ChemPhysChem, 13(5):1157-1160, feb 2012.

• Cooper-Pair Splitter: towards a source of source of entangled electrons
C. Schönenberger.
SPS Communications, 36(Jan):17-18, 2012.

### 2011

• Conductance fluctuations in graphene devices with superconducting contacts in different charge density regimes
F. Freitag, J. Trbovic, and C. Schönenberger.
Phys. Status Solidi B (arXiv:1108.4599), 248(11):2649-2652, oct 2011. arXiv:1108.4599
[Abstract]

Conductions fluctuations (CF) are studied in single layer graphene devices with superconducting source and drain contacts made from aluminium. The CF are found to be enhanced by superconductivity by a factor of 1.4–2. This (near) doubling of the CF indicates that the phase coherence length is equation image ≳ equation image. As compared to previous work, we find a relatively weak dependence of the CF on the gate voltage, and hence on the carrier density. We also demonstrate that whether the CF are larger or smaller at the charge neutrality point (CNP) can be strongly dependent on the series resistance RC, which needs to be subtracted.

• Finite bias Cooper pair splitting
L. Hofstetter, S. Csonka, A. Baumgartner, G. Fülöp, S. d’Hollosy, J. Nygård, and C. Schönenberger.
Phys Rev. Lett., 107(13):136801, sep 2011. arXiv:1105.2583
[Abstract]

In a device with a superconductor coupled to two parallel quantum dots (QDs) the electrical tunability of the QD levels can be used to exploit nonclassical current correlations due to the splitting of Cooper pairs. We experimentally investigate the effect of a finite potential difference across one quantum dot on the conductance through the other completely grounded QD in a Cooper pair splitter fabricated on an InAs nanowire. We demonstrate that the nonlocal electrical transport through the device can be tuned by electrical means and that the energy dependence of the effective density of states in the QDs is relevant for the rates of Cooper pair splitting (CPS) and elastic cotunneling. Such experimental tools are necessary to understand and develop CPS-based sources of entangled electrons in solid-state devices.

• Graphene Transistors Are Insensitive to pH Changes in Solution
W. Fu, C. Nef, O. Knopfmacher, A. Tarasov, M. Weiss, M. Calame, and C. Schönenberger.
Nano Letters, 11(9):3597-3600, sep 2011. arXiv:1105.0795
[Abstract]

We observe very small gate-voltage shifts in the transfer characteristic of as-prepared graphene field-effect transistors (GFETs) when the pH of the buffer is changed. This observation is in strong contrast to Si-based ion-sensitive FETs. The low gate-shift of a GFET can be further reduced if the graphene surface is covered with a hydrophobic fluorobenzene layer. If a thin Al-oxide layer is applied instead, the opposite happens. This suggests that clean graphene does not sense the chemical potential of protons. A GFET can therefore be used as a reference electrode in an aqueous electrolyte. Our finding sheds light on the large variety of pH-induced gate shifts that have been published for GFETs in the recent literature.

• Gate-tunable split Kondo effect in a carbon nanotube quantum dot
A. Eichler, M. Weiss, and C. Schönenberger.
Nanotechnology, 22(26):265204, may 2011. arXiv:1008.5103
[Abstract]

We show a detailed investigation of the split Kondo effect in a carbon nanotube quantum dot with multiple gate electrodes. Two conductance peaks, observed at finite bias in nonlinear transport measurements, are found to approach each other for increasing magnetic field, to result in a recovered zero bias Kondo resonance at finite magnetic field. Surprisingly, in the same charge state, but under different gate configurations, the splitting does not disappear for any value of the magnetic field, but we observe an avoided crossing. We think that our observations can be understood in terms of a two-impurity Kondo effect with two spins coupled antiferromagnetically. The exchange coupling between the two spins can be influenced by a local gate, and the non-recovery of the Kondo resonance for certain gate configurations is explained by the existence of a small antisymmetric contribution to the exchange interaction between the two spins

• Signal-to-noise ratio in dual-gated silicon nanoribbon field-effect sensors
A. Tarasov, W. Fu, O. Knopfmacher, J. Brunner, M. Calame, and C. Schönenberger.
Appl. Phys. Lett., 98(1):12114, jan 2011.
[Abstract]

Recent studies on nanoscale field-effect sensors reveal the crucial importance of the low-frequency noise for determining the ultimate detection limit. In this letter, the 1/f1/f-type noise of Si nanoribbon field-effect sensors is investigated. We demonstrate that the signal-to-noise ratio can be increased by almost two orders of magnitude if the nanoribbon is operated in an optimal gate voltage range. In this case, the additional noise contribution from the contact regions is minimized, and an accuracy of 0.5‰ of a pH shift in 1 Hz bandwidth can be reached.

### 2010

• Permalloy-based carbon nanotube spin-valve
H. Aurich, A. Baumgartner, F. Freitag, A. Eichler, J. Trbovic, and C. Schönenberger.
App. Phys. Lett, 97:153116, oct 2010. arXiv:1009.1960
[Abstract]

In this paper we demonstrate that permalloy (Py), a widely used Ni/Fe alloy, forms contacts to carbon nanotubes (CNTs) that meet the requirements for the injection and detection of spin-polarized currents in carbon-based spintronic devices. We establish the material quality and magnetization properties of Py strips in the shape of suitable electrical contacts and find a sharp magnetization switching tunable by geometry in the anisotropic magnetoresistance (AMR) of a single strip at cryogenic temperatures. In addition, we show that Py contacts couple strongly to CNTs, comparable to Pd contacts, thereby forming CNT quantum dots at low temperatures. These results form the basis for a Py-based CNT spin-valve exhibiting very sharp resistance switchings in the tunneling magnetoresistance, which directly correspond to the magnetization reversals in the individual contacts observed in AMR experiments.

• Hybrid superconductor – quantum dot devices
De S. Franceschi, L. Kouwenhoven, C. Schönenberger, and W. Wernsdorfer.
Nature Nanotechnology (invited), 5:703-711, sep 2010.
[Abstract]

Advances in nanofabrication techniques have made it possible to make devices in which superconducting electrodes are connected to non-superconducting nanostructures such as quantum dots. The properties of these hybrid devices result from a combination of a macroscopic quantum phenomenon involving large numbers of electrons (superconductivity) and the ability to control single electrons, offered by quantum dots. Here we review research into electron transport and other fundamental processes that have been studied in these devices. We also describe potential applications, such as a transistor in which the direction of a supercurrent can be reversed by adding just one electron to a quantum dot.

• Ferromagnetic Proximity Effect in a Ferromagnet Quantum-Dot Superconductor Device
L. Hofstetter, A. Geresdi, M. Aagesen, J. Nygård, C. Schönenberger, and S. Csonka.
Phys. Rev. Lett., 104:246804, jun 2010.
[Abstract]

The ferromagnetic proximity effect is studied in InAs nanowire based quantum dots strongly coupled to a ferromagnetic (F) and a superconducting (S) lead. The influence of the F lead is detected through the splitting of the spin-1=2 Kondo resonance. We show that the F lead induces a local exchange field on the quantum dot, which has varying amplitude and sign depending on the charge states. The interplay of the F and S correlations generates an exchange field related subgap feature.

• Magnetic field and contact resistance dependence of non-local charge imbalance
A. Kleine, A. Baumgartner, J. Trbovic, D. S. Golubev, A. D. Zaikin, and C. Schönenberger.
Nanotechnology, 21:274002, jun 2010. arXiv:0911.4427
[Abstract]

Crossed Andreev reflection (CAR) in metallic nanostructures, a possible basis for solid-state electron entangler devices, is usually investigated by detecting non-local voltages in multi-terminal superconductor/normal metal devices. This task is difficult because other subgap processes may mask the effects of CAR. One of these processes is the generation of charge imbalance (CI) and the diffusion of non-equilibrium quasi-particles in the superconductor. Here we demonstrate a characteristic dependence of non-local CI on a magnetic field applied parallel to the superconducting wire, which can be understood by a generalization of the standard description of CI to non-local experiments. These results can be used to distinguish CAR and CI and to extract CI relaxation times in superconducting nanostructures. In addition, we investigate the dependence of non-local CI on the resistance of the injector and detector contacts and demonstrate a quantitative agreement with a recent theory using only material and junction characteristics extracted from separate direct measurements.

• Superconductivity-enhanced conductance fluctuations in few layer graphene
J. Trbovic, N. Minder, F. Freitag, and C. Schönenberger.
Nanotechnology, 21:274005, jun 2010. arXiv:0912.0389
[Abstract]

We investigate the mesoscopic disorder induced rms conductance variance δG in short few-layer graphene (FLG) flakes contacted by two superconducting (S) Ti/Al contacts. By sweeping the back-gate voltage, we observe pronounced conductance fluctuations superimposed on a linear background of the two-terminal conductance G. The linear gate voltage induced response can be modelled by a set of interlayer and intralayer capacitances. δG depends on temperature T and source–drain voltage Vsd. δG increases with decreasing T and |Vsd|. When lowering |Vsd|, a pronounced cross-over at a voltage corresponding to the superconducting energy gap Δ is observed. For |V_{\mathrm {sd}}|\lesssim \Delta the fluctuations are markedly enhanced. Expressed in the conductance variance GGS of one graphene–superconductor (G–S) interface, values of 0.58e2/h are obtained at the base temperature of 230 mK. The conductance variance in the sub-gap region is larger by up to a factor of 1.4–1.8 compared to the normal state. The observed strong enhancement is due to phase coherent charge transfer caused by Andreev reflection at the G–S interface.

• Molecular junctions: from tunneling to function
M. Calame.
Chimia Int. J. Chem, 64:391-397, may 2010.
[Abstract]

Thanks to the development of appropriate experimental techniques, molecular devices and their electrical transport properties have recently been the focus of a major research effort. This brief review describes how individual molecules can be contacted with metallic electrodes to form molecular junctions and addresses their basic formation mechanisms. An extension to molecular junctions networks is also discussed. Functionality could be demonstrated in such systems, and examples where conductance modulation using light or chemical stimuli was achieved will be presented.

• The Nernst limit in dual-gated Si nanowire FET sensors
O. Knopfmacher, A. Tarasov, Wangyang Fu, M. Wipf, B. Niesen, M. Calame, and C. Schönenberger.
Nano Letters, 10(6):2268-2274, may 2010.
[Abstract]

Field effect transistors (FETs) are widely used for the label-free detection of analytes in chemical and biological experiments. Here we demonstrate that the apparent sensitivity of a dual-gated silicon nanowire FET to pH can go beyond the Nernst limit of 60 mV/pH at room temperature. This result can be explained by a simple capacitance model including all gates. The consistent and reproducible results build to a great extent on the hysteresis- and leakage-free operation. The dual-gate approach can be used to enhance small signals that are typical for bio- and chemical sensing at the nanoscale.

• Eine Trenneinrichtung für Quantenpaare
C. Schönenberger.
Physik in unserer Zeit, 41:58-59, mar 2010.
[Abstract]

Die Quantenmechanik erlaubt neben der Überlagerung von Zuständen auch deren Verschränkung. Das betrifft Teilchen mit Ruhemasse wie Elektronen ebenso wie masselose Photonen. Zwei Forscherteams ist es jüngst gelungen, verschränkte Elektronen in Form von Cooper-Paaren als Quelle verschränkter Elektronen zu nutzen. Dies eröffnet neue Möglichkeiten für grundlegende Experimente zur Quantenmechanik und könnte den Weg zur Quanteninformation auf einem Chip ebnen.

• Cyclic conductance switching in networks of redox-active molecular junctions
J. Liao, J. Agustsson, S. Wu, C. Schönenberger, M. Calame, Y. Leroux, M. Mayor, O. Jeannin, Y. -F. Ran, S. -X. Liu, and S. Decurtins.
Nano Letters, 10:759-764, feb 2010.
[Abstract]

Redox-active dithiolated tetrathiafulvalene derivatives (TTFdT) were inserted in two-dimensional nanoparticle arrays to build interlinked networks of molecular junctions. Upon oxidation of the TTFdT to the dication state, we observed a conductance increase of the networks by up to 1 order of magnitude. Successive oxidation and reduction cycles demonstrated a clear switching behavior of the molecular junction conductance. These results show the potential of interlinked nanoparticle arrays as chemical sensors.

• Novel Cruciform Structures as Model Compounds for Coordination Induced Single Molecule Switches
S. Grunder, R. Huber, S. Wu, C. Schönenberger, M. Calame, and M. Mayor.
Chimia Int. J. Chem, 64(3):140-144, jan 2010.
[Abstract]

We have synthesized various molecular cruciforms consisting of two different crossing n-systems and comprising crosswise arranged thiol- and pyridine-anchor groups. With these model compounds we strive towards the investigation of a new switching concept based on the potential dependent coordination of pyridines to gold electrodes in an electrochemical set-up. Integration of these cruciform molecules between both electrodes of a mechanically controlled break junction in a liquid environment gave insight into their single molecule transport properties. These studies allowed individual transport characteristics to be assigned to the bar subunits of the cruciforms but also revealed the remaining experimental challenges to realize the suggested switching concept.

• Oligoaryl Cruciform Structures as Model Compounds for Coordination-Induced Single-Molecule Switches
S. Grunder, R. Huber, S. Wu, C. Schönenberger, M. Calame, and M. Mayor.
Eur. J. Org. Chem., 2010:833�845, jan 2010.
[Abstract]

The synthesis of two new cruciform structures 3 and 4 comprising an oligo(phenylene-vinylene) (OPV) and a perpendicular oligoaryl bar – namely an oligophenylene (OP) (3) and a naphthyl–phenyl–naphthyl system (4) – is reported. The OPV rod consists of two terminal pyridine units, whereas the oligoaryl rod bears two terminal acetylsulfanyl groups as protected anchor groups. The OPV bar was assembled via a Horner–Wadsworth–Emmons reaction, which directly led to the desired E,E isomers. The perpendicular oligoaryl bars were assembled with a Suzuki reaction using the corresponding boronic acids, which were already fitted with an ethyl-trimethylsilane (ethyl-TMS) sulfanyl group. In a last step, the ethyl-TMS-protected sulfur atoms were transprotected to the thioacetyl units. The cruciform structures 3 and 4 are model compounds to investigate a coordination-induced single-molecule switch exploiting the potential-dependant different bonding strengths of the anchor groups to gold. Metal–molecule–metal junctions were formed using a mechanically controllable break junction (MCBJ) setup. Current traces of molecular junctions were statistically analyzed. Further investigations of model compounds consisting only of the single bar confirm that individual molecules carrying the required function for the switching experiments were trapped between two electrodes and were mainly immobilized via thiol–gold anchor bonds.

• Cooper Pair Splitter: Eine Trenneinrichtung für Quantenpaare
C. Schönenberger.
SSOM Bulletin 1+2 2010, 41:58-59, 2010.

### 2009

• Cooper pair splitter realized in a two-quantum-dot Y-junction
L. Hofstetter, S. Csonka, J. Nygård, and C. Schönenberger.
Nature, 461:960-963, aug 2009.
[Abstract]

Non-locality is a fundamental property of quantum mechanics that manifests itself as correlations between spatially separated parts of a quantum system. A fundamental route for the exploration of such phenomena is the generation of Einstein–Podolsky–Rosen (EPR) pairs of quantum-entangled objects for the test of so-called Bell inequalities. Whereas such experimental tests of non-locality have been successfully conducted with pairwise entangled photons, it has not yet been possible to realize an electronic analogue of it in the solid state, where spin-1/2 mobile electrons are the natural quantum objects3. The difficulty stems from the fact that electrons are immersed in a macroscopic ground state—the Fermi sea—which prevents the straightforward generation and splitting of entangled pairs of electrons on demand. A superconductor, however, could act as a source of EPR pairs of electrons, because its ground-state is composed of Cooper pairs in a spin-singlet state. These Cooper pairs can be extracted from a superconductor by tunnelling, but, to obtain an efficient EPR source of entangled electrons, the splitting of the Cooper pairs into separate electrons has to be enforced. This can be achieved by having the electrons ‘repel’ each other by Coulomb interaction. Controlled Cooper pair splitting can thereby be realized by coupling of the superconductor to two normal metal drain contacts by means of individually tunable quantum dots. Here we demonstrate the first experimental realization of such a tunable Cooper pair splitter, which shows a surprisingly high efficiency. Our findings open a route towards a first test of the EPR paradox and Bell inequalities in the solid state.

• Finite-bias visibility dependence in an electronic Mach-Zehnder interferometer
E. Bieri, M. Weiss, O. Göktas, M. Hauser, C. Schönenberger, and S. Oberholzer.
Phys. Rev. B, 79:245324, jun 2009.
[Abstract]

We use an electronic Mach-Zehnder interferometer to explore the nonequilibrium coherence of the electron waves within the edge states that form in the integral quantum-Hall-effect device. The visibility of the interference as a function of bias voltage and transmission probabilities of the mirrors, which are realized by quantum point contacts, reveals an unexpected asymmetry at finite bias when the transmission probability T of the mirror at the input of the interferometer is varied between 0 and 100%, while the transmission probability of the other mirror at the output is kept fixed. This can lead to the surprising result of an increasing magnitude of interference with increasing bias voltage for certain values of T. A detailed analysis for various transmission probabilities and different directions of the magnetic field demonstrates that this effect is not related to the transmission characteristics of a single-quantum point contact but is an inherent property of the Mach-Zehnder interferometer with edge states.

• Surface plasmon enhanced photoconductance of gold nanoparticle arrays with incorporated alkane linkers
M. A. Mangold, C. Weiss, M. Calame, and A. W. Holleitner.
Applied Physics Letters, 94(16):161104, apr 2009. arXiv:0902.4807
[Abstract]

We report on a photoconductive gain effect in two-dimensional arrays of gold nanoparticles, in which alkane molecules are inserted. The nanoparticle arrays are formed by a self-assembly process from alkanethiol-coated gold nanoparticles, and subsequently they are patterned on a Si/SiO2 chip by a microcontact printing technique. We find that the photoconductance of the arrays is strongly enhanced at the frequency of the surface plasmon of the nanoparticles. We interpret the observation as a bolometric enhancement of the conductance of the nanoparticle arrays upon excitation of the surface plasmon resonance.

• Tuning the Josephson current in carbon nanotubes with the Kondo effect
A. Eichler, R. Deblock, M. Weiss, C. Schönenberger, H. Bouchiat, C. Karrasch, and V. Meden.
Phys. Rev. B, 79:161407(R), apr 2009. arXiv:0810.1671
[Abstract]

We investigate the Josephson current in a single wall carbon nanotube connected to superconducting electrodes. We focus on the parameter regime in which transport is dominated by Kondo physics. A sizeable supercurrent is observed for odd number of electrons on the nanotube when the Kondo temperature TK is sufficiently large compared to the superconducting gap. On the other hand when, in the center of the Kondo ridge, TK is slightly smaller than the superconducting gap, the supercurrent is found to be extremely sensitive to the gate voltage V_BG. Whereas it is largely suppressed at the center of the ridge, it shows a sharp increase at a finite value of V_BG. This increase can be attributed to a doublet-singlet transition of the spin state of the nanotube island leading to a π shift in the current phase relation. This transition is very sensitive to the asymmetry of the contacts and is in good agreement with theoretical predictions.

• Light-controlled conductance switching of ordered metal-molecule-metal devices
S. J. van der Molen, J. Lia, T. Kudernac, J. Agustsson, L. Bernard, M. Calame, B. van Wees, B. L. Fering, and C. Schönenberger.
Nano Letters, 9:76-80, jan 2009.
[Abstract]

We demonstrate reversible, light-controlled conductance switching of molecular devices based on photochromic diarylethene molecules. These devices consist of ordered, two-dimensional lattices of gold nanoparticles, in which neighboring particles are bridged by switchable molecules. We independently confirm that reversible isomerization of the diarylethenes employed is at the heart of the room-temperature conductance switching. For this, we take full advantage of the possibility to use optical spectroscopy to follow molecular switching in these samples.

• Contact resistance dependence of crossed Andreev reflection
A. Kleine, A. Baumgartner, J. Trbovic, and C. Schönenberger.
Eur. Phys. Lett., 87:27011, 2009. arXiv:0812.3553
[Abstract]

We report experiments in nanometer-scaled superconductor/normal metal hybrid devices which show that in a small window of contact resistances, crossed Andreev reflection (CAR) can dominate the nonlocal transport for all energies below the superconducting gap. Besides crossed Andreev reflection, elastic cotunneling (EC) and nonlocal charge imbalance can be identified as competing subgap transport mechanisms in temperature-dependent four-terminal nonlocal measurements. We demonstrate a systematic change of the nonlocal resistance vs. bias characteristics with increasing contact resistances, which can be varied in the fabrication process. For samples with higher contact resistances, CAR is weakened relative to EC in the midgap regime, possibly due to dynamical Coulomb blockade. Gaining control of crossed Andreev reflection is an important step towards the realization of a solid-state entangler.

• Correlated electron systems: Gap opens in metallic nanotubes
C. Schönenberger.
Nature Nanotechnology, 4:147-148, 2009.

### 2008

• Electrical conductance of conjugated oligomers at the single molecule level
R. Huber, M. -T. Gonzalez, S. Wu, M. Langer, S. Grunder, V. Horhoiu, M. Mayor, M. Bryce, C. Wang, R. Jitchati, C. Schönenberger, and M. Calame.
J. Am. Chem. Soc., 130:1080-1084, dec 2008.
[Abstract]

We determine and compare, at the single molecule level and under identical environmental conditions, the electrical conductance of four conjugated phenylene oligomers comprising terminal sulfur anchor groups with simple structural and conjugation variations. The comparison shows that the conductance of oligo(phenylene vinylene) (OPV) is slightly higher than that of oligo(phenylene ethynylene) (OPE). We find that solubilizing side groups do neither prevent the molecules from being anchored within a break junction nor noticeably influence the conductance value.

• Scaling of 1/f noise in tunable break-junctions
ZhengMing Wu, SongMei Wu, S. Oberholzer, M. Steinacher, M. Calame, and C. Schönenberger.
Phys. Rev. B, 78:235421, dec 2008. arXiv:0809.4841
[Abstract]

We have studied the 1/f voltage noise of gold nanocontacts in electromigrated and mechanically controlled break junctions having resistance values R that can be tuned from 10 Ohm (many channels) to 10 kOhm (single atom contact). The noise is caused by resistance fluctuations as evidenced by the SV proprtional V^2 dependence of the power-spectral density SV on the applied dc voltage V. As a function of R the normalized noise SV/V^2 shows a pronounced crossover from R^3 for low-Ohmic junctions to R^1.5 for high-Ohmic ones. The measured powers of 3 and 1.5 are in agreement with 1/f noise generated in the bulk and reflect the transition from diffusive to ballistic transport.

• Giant g-factor fluctuations in InAs Nanowire Quantum Dots
S. Csonka, L. Hofstetter, F. Freitag, S. Oberholzer, C. Schönenberger, T. S. Jespersen, M. Aagesen, and J. Nygård.
Nano Letters, 8:3932-3935, oct 2008. arXiv:0808.1492
[Abstract]

We study the g-factor of discrete electron states in InAs nanowire based quantum dots. The g values are determined from the magnetic field splitting of the zero bias anomaly due to the spin 1/2 Kondo effect. Unlike to previous studies based on 2DEG quantum dots, the g-factors of neighboring electron states show a surprisingly large fluctuation: g can scatter between 2 and 18. Furthermore electric gate tunability of the g-factor is demonstrated.

• Molecular junctions based on aromatic coupling
S. Wu, M. T. Gonzalez, R. Huber, S. Grunder, M. Mayor, C. Schönenberger, and M. Calame.
Nature Nanotechnology, 3:569-574, aug 2008.
[Abstract]

If individual molecules are to be used as building blocks for electronic devices, it will be essential to understand charge transport at the level of single molecules. Most existing experiments rely on the synthesis of functional rod-like molecules with chemical linker groups at both ends to provide strong, covalent anchoring to the source and drain contacts. This approach has proved very successful, providing quantitative measures of single-molecule conductance, and demonstrating rectification and switching at the single-molecule level. However, the influence of intermolecular interactions on the formation and operation of molecular junctions has been overlooked. Here we report the use of oligo-phenylene ethynylene molecules as a model system, and establish that molecular junctions can still form when one of the chemical linker groups is displaced or even fully removed. Our results demonstrate that aromatic π−π coupling between adjacent molecules is efficient enough to allow for the controlled formation of molecular bridges between nearby electrodes.

• Conductance values of alkanethiol molecular junctions
M. T. Gonzalez, J. Brunner, R. Huber, S. Wu, C. Schönenberger, and M. Calame.
New J. of Phys., 10:65018, jun 2008.
[Abstract]

We study the electrical conductance of octanedithiol molecular junctions using a mechanically controllable break-junction setup. The stability of the system allows control of whether the electrodes get into contact before each new molecular junction formation or not (contact and non-contact modes). We find three characteristic conductance values for octanedithiol. Well-defined peaks in the conductance histograms at multiples of 1.2×10−5 G0 suggest that this value corresponds to the conductance of a single molecular junction conductance. Reproducible features are also observed at 4.5×10−5 and 2.3×10−4 G0. The first value has the strongest statistical weight, whereas the second is only observed in the non-contact mode. We propose that these two values reflect the formation of several molecular junctions in parallel between the electrodes.

• Interlinking Au nanoparticles in 2D arrays via conjugated dithiolated molecules
J. Liao, Markus A. Mangold, S. Grunder, M. Mayor, C. Schönenberger, and M. Calame.
New J. of Phys., 10:65019, jun 2008.
[Abstract]

We investigate the importance of anchoring end-groups in conjugated oligomers for the formation of molecular junction networks. Oligo(phenylene ethynylene) with a single (OPE-MT) and two (OPE-DT) thiol end-groups have been inserted into self-assembled octanethiol-capped gold nanoparticle arrays by taking advantage of molecular exchange. Comparing the exchange for tens of devices, we observe significantly different final conductances for devices comprising monothiol- and dithiolated compounds. Our experimental results support the picture that OPE-DT covalently bridge neighboring nanoparticles via Au–S bonds at both ends of the conjugated oligomer to form interlinked networks of molecular junctions.

• Detection of Transient Events in the Presence of Background Noise
Wilfried Grange, Philippe Haas, Andreas Wild, Michael Andreas Lieb, Michel Calame, Martin Hegner, and Bert Hecht.
J. Phys. Chem. B, 112(23):7140-7144, may 2008.
[Abstract]

We describe a method to detect and count transient burstlike signals in the presence of a significant stationary noise. To discriminate a transient signal from the background noise, an optimum threshold is determined using an iterative algorithm that yields the probability distribution of the background noise. Knowledge of the probability distribution of the noise then allows the determination of the number of transient events with a quantifiable error (wrong-positives). We apply the method, which does not rely on the choice of free parameters, to the detection and counting of transient single-molecule fluorescence events in the presence of a strong background noise. The method will be of importance in various ultra sensing applications.

• Large oscillating non-local voltage in multiterminal single-wall carbon nanotube devices
G. Gunnarsson, J. Trbovic, and C. Schönenberger.
Phys. Rev. B (rapid), 77:201405, may 2008. arXiv:0710.0365
[Abstract]

We report on the observation of a nonlocal voltage in a ballistic (quasi)-one-dimensional conductor, realized by a single-wall carbon nanotube with four contacts. The contacts divide the tube into three quantum dots, which we control by the back-gate voltage Vg . We measure a large oscillating nonlocal voltage Vnl as a function of Vg . Though a resistor model that includes the impedance of the voltmeter can account for a nonlocal voltage including change of sign, it fails to describe the magnitude properly. The large amplitude of Vnl is due to quantum interference effects and can be understood within the scattering approach of electron transport.

### 2007

• Spectroscopy of Molecular Junction Networks Obtained by Place Exchange in 2D Nanoparticle Arrays
L. Bernard, Y. Kamdzhilov, M. Calame, S. J. van der Molen, J. Liao, and C. Schönenberger.
J. Phys. Chem. C, 111:18445-18450, nov 2007.
[Abstract]

Well ordered nanoparticle arrays were prepared on Si/SiO2 surfaces from alkanethiol-coated Au nanoparticles via self-assembly and micro-contact printing. We study the insertion of conjugated molecular species within the nanoparticle arrays via spectroscopic and electrical transport measurements. Upon exchange of the alkanethiol chains with the conjugated oligomers, the conductance of the network increases by one to 3 orders of magnitude. In addition, the absorption spectra in the visible light range show a red-shift of the surface plasmon resonance (SPR). The latter shift, which is due to the difference in permittivity between alkanes and conjugated oligomers, can be understood within Mie and Maxwell−Garnett theory. Finally, infrared absorption spectra provide direct spectroscopic evidence that the conjugated oligomers can be not only inserted but also, subsequently, fully removed from the nanoparticle arrays via place-exchange. The reversibility of the exchange process demonstrates the potential of these structures as a platform for molecular electronics.

• Tetrathiafulvalene-based molecular nanowires
F. Giacalome, M. A. Herranz, L. Grüter, M. T. Gonzalez, M. Calame, C. Schönenberger, C. R. Arroyo, G. Rubio-Bollinger, M. Vélez3, N. Agrait, and N. Martin.
ChemComm, pages 4854-4856, nov 2007.
[Abstract]

A new molecular wire suitably functionalized with sulfur atoms at terminal positions and endowed with a central redox active TTF unit has been synthesized and inserted within two atomic-sized Au electrodes; electrical transport measurements have been performed in STM and MCBJ set-ups in a liquid environment and reveal conductance values around 10–2G0 for a single molecule.

• Mapping electron delocalization by charge transport spectroscopy in an artificial molecule
M. R. Gräber, M. Weiss, D. Keller, S. Oberholzer, and C. Schönenberger.
Annalen der Physik, 16(10-11):672-677, oct 2007. arXiv:0705.3962
[Abstract]

In this letter we present an experimental realization of the quantum mechanics textbook example of two interacting electronic quantum states that hybridize forming a molecular state. In our particular realization, the quantum states themselves are fabricated as quantum dots in a molecule, a carbon nanotube. For sufficient quantum-mechanical interaction (tunnel coupling) between the two quantum states, the molecular wavefunction is a superposition of the two isolated (dot) wavefunctions. As a result, the electron becomes delocalized and a covalent bond forms. In this work, we show that electrical transport can be used as a sensitive probe to measure the relative weight of the two components in the superposition state as a function of the gate-voltages. For the field of carbon nanotube double quantum dots, the findings represent an additional step towards the engineering of quantum states.

• New Cruciform Structures: Toward Coordination Induced Single Molecule Switches
S. Grunder, R. Huber, V. Horhoiu, M. -T. Gonzalez, C. Schönenberger, M. Calame, and M. Mayor.
J. Org. Chem, 72:8337-8344, oct 2007.
[Abstract]

New cruciform structures 1−4 were synthesized to investigate a new single molecule switching mechanism arising from the interplay between the molecule and the electrode surface. These molecular cruxes consist of two rod-type substructures, namely an oligophenylenevinylene and an oligophenyleneethynyl. While the oligophenylenevinylene rods are functionalized with acetyl protected sulfur anchor groups, the oligophenyleneethynyl rods provide terminal pyridine units. The hypothesized switching mechanism should arise from the electrochemical potential dependent coordination of the pyridine unit to the electrode surface. The assembly of the oligophenylenevinylene substructure was based on a Wittig reaction whereas its perpendicular oligophenyleneethynyl rod was assembled by Sonogashira−Hagihara coupling reactions. Preliminary transport investigations with molecular cruciforms 2 and 4 in a mechanical controllable break junction in a liquid environment displayed the trapping of single molecules between two gold electrodes via the terminally sulfur functionalized oligophenylenevinylene rod.

• Even-Odd Effect in Andreev Transport through a Carbon Nanotube Quantum Dot
A. Eichler, M. Weiss, S. Oberholzer, C. Schönenberger, Levy A. Yeyati, J. C. Cuevas, and A. Martin-Rodero.
Phys. Rev. Lett., 99:126602, sep 2007. arXiv:0703082
[Abstract]

We have measured the current($I$)-voltage($V$) characteristics of a single-wall carbon nanotube quantum dot coupled to superconducting source and drain contacts in the intermediate coupling regime. Whereas the enhanced differential conductance $dI/dV$ due to the Kondo resonance is observed in the normal state, this feature around zero bias voltage is absent in the super\-conducting state. Nonetheless, a pronounced even-odd effect appears at finite bias in the $dI/dV$ sub-gap structure caused by Andreev reflection. The first-order Andreev peak appearing around $V=\Delta/e$ is markedly enhanced in gate-voltage regions, in which the charge state of the quantum dot is odd. This enhancement is explained by a hidden’ Kondo resonance, pinned to one contact only. A comparison with a single-impurity Anderson model, which is solved numerically in a slave-boson mean\-field approach, yields good agreement with the experiment.

• Feedback controlled electromigration in four-terminal nanojunctions
Z. -M. Wu, M. Steinacher, R. Huber, M. Calame, S. J. Molen, and C. Schönenberger.
Appl. Phys. Lett., 91:53118, aug 2007.
[Abstract]

The authors have developed a fast, yet highly reproducible method to fabricate metallic electrodes with nanometer separation using electromigration (EM). They employ four terminal instead of two-terminal devices in combination with an analog feedback to maintain the voltage UU over the junction constant. After the initialization phase (U < 0.2V), during which the temperature T increases by 80–150°C, EM sets in shrinking the wire locally. This quickly leads to a transition from the diffusive to a quasiballistic regime (0.2V < U < 0.6V). At the end of this second regime, a gap forms (U > 0.6V). Remarkably, controlled electromigration is still possible in the quasiballistic regime.

• Controlled formation of metallic nanowires via Au nanoparticle ac trapping
L. Bernard, M. Calame, S. J. van der Molen, J. Liao, and C. Schönenberger.
Nanotechnology, 18:235202, may 2007.
[Abstract]

Applying ac voltages, we trapped gold nanoparticles between micro-fabricated electrodes under well-defined conditions. We demonstrate that the nanoparticles can be controllably fused together to form homogeneous gold nanowires with pre-defined diameters and conductance values. Whereas electromigration is known to form a gap when a dc voltage is applied, this ac technique achieves the opposite, thereby completing the toolkit for the fabrication of nanoscale junctions.

### 2006

• Nanospintronics with Carbon Nanotubes
A. Cottet, T. Kontos, S. Sahoo, H. T. Man, M. -S. Choi, W. Belzig, C. Bruder, A. F. Morpurgo, and C. Scönenberger.
Semicond. Sci. Technol., 21:S78-S95, oct 2006. arXiv:0703472
[Abstract]

One of the actual challenges of spintronics is the realization of a spin transistor allowing control of spin transport through an electrostatic gate. In this paper, we report on different experiments which demonstrate gate control of spin transport in a carbon nanotube connected to ferromagnetic leads. We also discuss some theoretical approaches which can be used to analyse spin transport in these systems. We emphasize the roles of the gate-tunable quasi-bound states inside the nanotube and the coherent spin-dependent scattering at the interfaces between the nanotube and its ferromagnetic contacts.

• Defining and Controlling Double Quantum Dots in Single-Wall Carbon Nanotubes
M. R. Gräber, M. Weiss, S. Oberholzer, and C. Schönenberger.
Semicond. Sci. Technol., 21:S64-S68, oct 2006. arXiv:0605220
[Abstract]

We report the experimental realization of double quantum dots in single-walled carbon nanotubes. The device consists of a nanotube with source and drain contact, and three additional top-gate electrodes in between. We show that, by energizing these top gates, it is possible to locally gate a nanotube, to create a barrier, or to tune the chemical potential of a part of the nanotube. At low temperatures, we find (for three different devices) that in certain ranges of top-gate voltages our device acts as a double quantum dot, evidenced by the typical honeycomb charge stability pattern.

• Reversible formation of molecular junctions in two-dimensional nanoparticle arrays
J. Liao, L. Bernard, M. Langer, C. Schönenberger, and M. Calame.

• Charge and Spin Transport in Carbon Nanotubes
C. Schönenberger.
Semicond. Sci. Technol., 21:S1-S9, oct 2006.
[Abstract]

The basic science in quantum transport of nano-scaled ‘devices’ is largely based on the availability of suitable model systems. Nanostructures built from conventional metals are typically in the diffusive transport regime. Semiconductors, as the starting material for nanodevices, are different. Because of the low carrier density and therefore reduced screening, the Fermi energy can be tuned by electrostatic gates. Quantum dots which can be filled sequentially with electrons one by one have been realized in this material system (for a review see Kouwenhoven et al (2001 Rep. Prog. Phys. 64 701)). Today, researchers have also started to explore the new possibilities provided by molecules (see, for example, Selzer and Allara (2006 Ann. Rev. Phys. Chem. 57 593), Cuniberti et al (2006 Lecture Notes in Physics vol 680), McCreery (2004 Chem. Mater. 16 4477)). A rather simple prototype ‘molecule’ is a carbon nanotube (CNT) (for recent reviews, see Anantram and Leonard (2006 Rep. Prog. Phys. 69 507), Dresselhaus et al (2001 Topics in Applied Physics vol 80), Ebbesen (1996 Phys. Today 49 26)). Charge and spin transport in CNTs have attracted a lot of attention in recent years. There are several reasons for this excitement: CNTs are almost ideal quantum-ballistic wires. Large electric field effects have been observed in semiconducting CNTs, potentially of interest for applications in electronics. Because a CNT is an all-surface conductor, the electrical properties are highly sensitive to the environment, which can be exploited in sensing applications. Finally, a wealth of new physics is currently appearing in experiments in which CNT-hybrid devices are used, which employ a combination of normal metal, superconducting and ferromagnetic contacts.

• Electrical Conductance of Molecular Junctions by a Robust Statistical Analaysis
M. T. González, S. Wu, R. Huber, S. J. vam der Molen, C. Schönenberger, and M. Calame.
Nano Letters, 6(10):2238-2242, sep 2006. arXiv:0610303
[Abstract]

We propose an objective and robust method to extract the electrical conductance of single molecules connected to metal electrodes from a set of measured conductance data. Our method roots in the physics of tunneling and is tested on octanedithiol using mechanically controllable break junctions. The single molecule conductance values can be deduced without the need for data selection.

• Directional scrolling of hetero-films on Si (110) and Si (111) surfaces
L. Zhang, E. Deckhardt, A. Weber, C. Schönenberger, and D. Grützmacher.
Microelectronic Engineering, 83(4-9):1233-1236, sep 2006.
[Abstract]

Freestanding micro-tubes were obtained by directional scrolling of SiGe/Si/Cr and SiGe/Si hybrid- and hetero-structures on Si(1 1 0) and Si (1 1 1) substrates. In addition, on a Si(1 1 1) surface, helical nanobelts and vertical structure were achieved too. Compared to a Si(0 0 1) substrate, hetero-films on Si(1 1 0) and Si(1 1 1) substrates present different preferred scrolling directions for tubes resulting from the strong anisotropic underetching rate in directions parallel to the substrates surface. The design rules for the fabrication of micro- and nanotubes on Si(1 1 0) and Si(1 1 1) substrates are elucidated. The flexibility in the design is promising for applications in micro- and nano-electromechanics.

• Fabrication and characterization of freestanding Si/Cr micro- and nanospirals
L. Zhang, L. Dong, D. J. Bell, B. J. Nelson, C. Schönenberger, and D. Grützmacher.
Microelectronic Engineering, 83(4-9):1237-40, sep 2006.
[Abstract]

We report on the scrolling of two-layer Si/Cr hybrid films on Si(0 0 1) wafers to form rolled-up micro-and nanostructures. Mesa stripes with width ranging from 3.0 μm to 60 nm have been used to fabricate the spirals. Decreasing the width gradually changes the scrolling direction of the Si/Cr stripe from the <1 0 0> direction to the longitudinal axis of the stripe. Moreover, the diameter of the Si/Cr rings decreases significantly with decreasing stripe width, which can be explained by edge effects at the side walls of the bilayer film. Based on this effect, freestanding Si/Cr spiral nanobelts can be fabricated. The flexibility of such spirals has been probed with a nano-manipulator showing their excellent shape-memory properties.

• Molecular States in Carbon Nanotube Double Quantum Dots
M. R. Gräber, W. A. Coish, C. Hoffmann, M. Weiss, J. Furer, S. Oberholzer, D. Loss, and C. Schönenberger.
Phys. Rev. B, 74:75427, aug 2006. arXiv:0603367
[Abstract]

We report electrical transport measurements through a semiconducting single-walled carbon nanotube with three additional top gates. At low temperatures the system acts as a double quantum dot with large interdot tunnel coupling allowing for the observation of tunnel-coupled molecular states extending over the whole double-dot system. We precisely extract the tunnel coupling and identify the molecular states by the sequential-tunneling line shape of the resonances in differential conductance.

• Schaltende Moleküle
C. Schönenberger, M. Calame, and M. Mayor.
UniNova, Wissenschaftsmagazin der Universität Basel, 103:22-24, jul 2006.

• Molecular Electronics
M. Calame and C. Schönenberger.
Imaging & Microscopy, 8:36, Jun 2006.

• Anomalous Coiling of SiGe/Si and SiGe/Si/Cr Helical Nanobelts
L. Zhang, E. Ruh, D. Grützmacher, L. Dong, D. J. Bell, B. J. Nelson, and C. Schönenberger.
Nano Letters, 6(7):1311-1317, jun 2006.
[Abstract]

The fabrication of nanohelices by the scrolling of strained bilayers is investigated. It is shown that structure design is dominated by edge effects rather than bulk crystal properties such as the Young’s modulus when the dimensions of the structures are reduced below 400 nm. SiGe/Si/Cr, SiGe/Si, and Si/Cr helical nanobelts are used as test structures. Dimensions of the belt width are reduced from 1.30 μm to 300 nm, and parameters controlling helicity angle, chirality, diameter, and pitch of the nanohelices are investigated. An anomalous scrolling direction deviating from the preferred <100> scrolling direction has been found for small structures. Making use of the anomalous scrolling, it is possible to fabricate three-dimensional helices with helicity angles less than 45°, which is advantageous for micro- and nanoelectromechanical systems.

• Controlling spin in an electronic interferometer with spin-active interfaces
A. Cottet, T. Kontos, W. Belzig, C. Schönenberger, and C. Bruder.
Europhys. Lett., 74:320-326, mar 2006. arXiv:0512176
[Abstract]

We consider electronic current transport through a ballistic one-dimensional quantum wire connected to two ferromagnetic leads. We study the effects of the spin-dependence of interfacial phase shifts (SDIPS) acquired by electrons upon scattering at the boundaries of the wire. The SDIPS produces a spin-splitting of the wire resonant energies which is tunable with the gate voltage and the angle between the ferromagnetic polarizations. This property could be used for manipulating spins. In particular, it leads to a giant magnetoresistance effect with a sign tunable with the gate voltage and the magnetic field applied to the wire.

• Positive cross-correlations in a normal-conducting fermionic beam-splitter
S. Oberholzer, E. Bieri, C. Schönenberger, M. Giovannini, and J. Faist.
Phys. Rev. Lett., 96:46804, feb 2006. arXiv:0510240
[Abstract]

We investigate a beam-splitter experiment implemented in a normal-conducting fermionic electron gas in the quantum Hall regime. The cross correlations between the current fluctuations in the two exit leads of the three terminal device are found to be negative, zero, or even positive, depending on the scattering mechanism within the device. Reversal of the cross correlation sign occurs due to interaction between different edge states and does not reflect the statistics of the fermionic particles which “antibunch.”

### 2005

• Electric field control of spin transport
S. Sahoo, T. Kontos, J. Furer, C. Hoffmann, M. Gräber, A. Cottet, and C. Schönenberger.
Nature Physics, 1:99-102, nov 2005. arXiv:0511078
[Abstract]

Spintronics aims to develop electronic devices whose resistance is controlled by the spin of the charge carriers that flow through them1, 2, 3. This approach is illustrated by the operation of the most basic spintronic device, the spin valve4, 5, 6, which can be formed if two ferromagnetic electrodes are separated by a thin tunnelling barrier. In most cases, its resistance is greater when the two electrodes are magnetized in opposite directions than when they are magnetized in the same direction7, 8. The relative difference in resistance, the so-called magnetoresistance, is then positive. However, if the transport of carriers inside the device is spin- or energy-dependent3, the opposite can occur and the magnetoresistance is negative9. The next step is to construct an analogous device to a field-effect transistor by using this effect to control spin transport and magnetoresistance with a voltage applied to a gate10, 11. In practice though, implementing such a device has proved difficult. Here, we report on a pronounced gate-field-controlled magnetoresistance response in carbon nanotubes connected by ferromagnetic leads. Both the magnitude and the sign of the magnetoresistance in the resulting devices can be tuned in a predictable manner. This opens an important route to the realization of multifunctional spintronic devices.

• Electrical conductance of atomic contacts in liquid envirnoments
L. Grüter, M. T. González, R. Huber, Michel C., and C. Schönenberger.
Small, 1(11):1067-1070, sep 2005.
[Abstract]

Mechanically controllable break junctions have been characterized in different solvents as well as in air and vacuum. In the high-conductance regime, the environment plays a minor role, while in the low-conductance (tunneling) regime, a systematic and reproducible lowering of the tunneling barrier height is observed.

• Resonant tunnelling through a C60 molecular junction in a liquid environment
L. Grüter, F. Cheng, T. T. Heikkilä, M. T. González, F. Diederich, C. Schönenberger, and M. Calame.
Nanotechnology, 16:2143-2148, aug 2005. arXiv:0507264
[Abstract]

We present electronic transport measurements through thiolated C60 molecules in a liquid environment. The molecules were placed within a mechanically controllable break junction using a single anchoring group per molecule. On varying the electrode separation of the C60-modified junctions, we observed a peak in the conductance traces. The shape of the curves is strongly influenced by the environment of the junction as shown by measurements in two distinct solvents. In the framework of a simple resonant tunnelling model, we can extract the electronic tunnelling rates governing the transport properties of the junctions.

• Shot-noise and conductance measurements of transparent superconductor/two-dimensional electron gas junctions
B. -R. Choi, A. E. Hansen, T. Kontos, C. Hoffmann, S. Oberholzer, W. Belzig, C. Schönenberger, T. Akazaki, and H. Takayanagi.
Phys. Rev. B, 72:24501, jul 2005. arXiv:0410621
[Abstract]

We have measured the conductance and shot noise of superconductor-normal metal S-N junctions between a niobium Nb film and a two-dimensional electron gas 2DEG, formed in an InAs-based semiconductor heterostructure. Adjacent to the junction, the 2DEG is shaped into a sub-micrometer beam splitter. The current shot noise measured through one arm of the beam splitter is found to be enhanced due to Andreev reflection. Both noise and conductance measurements indicate that the Nb-2DEG interface is of high quality with a transparency approaching 60–70 %. The present device can be seen as a quasi-ballistic S-N beam-splitter junction.

• Electrical spin injection in multi-wall carbon nanotubes with transparent ferromagnetic contacts
S. Sahoo, T. Kontos, C. Schönenberger, and C. Sürgers.
Appl. Phys. Lett., 86:112109, mar 2005. arXiv:0411623
[Abstract]

We report on electrical spin injection measurements on multiwall carbon nanotubes (MWNTs). We use a ferromagnetic alloy Pd1−xNixPd1−xNix with x≈0.7x≈0.7 which allows us to obtain devices with resistances as low as 5.6kΩ5.6kΩ at 300 K. The yield of device resistances below 100kΩ100kΩ, at 300 K, is around 50%. We measure at 2 K a hysteretic magneto-resistance due to the magnetization reversal of the ferromagnetic leads. The relative difference between the resistance in the antiparallel (AP)(AP) orientation and the parallel (P)(P) orientation is about 2%.

• Controllable fabrication of SiGe/si and SiGe/Si/Cr helical nanobelts
L. Zhang, E. Deckhardt, A. Weber, C. Schönenberger, and D. Grützmacher.
Nanotechnology, 16:655-663, mar 2005.
[Abstract]

Helical nanobelts of SiGe/Si and SiGe/Si/Cr are fabricated by rolling up strained thin heterostructures. The fabrication involved electron beam lithography, reactive ion etching, and wet chemical etching steps followed by a drying procedure. All parameters of the helical nanobelts, namely their helical angle, chirality, pitch and diameter, are controllable in a reproducible fashion. The ease of fabrication of SiGe/Si and hybrid helical nanobelts opens new paths for the fabrication technology of micro- or nanoscale sensors, transducers, resonators and cylindrical shaped micro-capacitors.

### 2004

• Kondo effect in carbon nanotubes at half-filling
B. Babić, T. Kontos, and C. Schönenberger.
Phys. Rev. B, 70:235419, dec 2004. arXiv:0407193
[Abstract]

{In a single state of a quantum dot the Kondo effect arises due to the spin-degeneracy, which is present if the dot is occupied with one electron (N = 1). The eigenstates of a carbon nanotube quantum dot possess an additional orbital degeneracy leading to a four-fold shell pattern. This additional degeneracy increases the possibility for the Kondo effect to appear. We revisit the Kondo problem in metallic carbon nanotubes by linear and non-linear transport measurement in this regime, in which the four-fold pattern is present. We have analyzed the ground state of CNTs, which were grown by chemical vapor deposition, at filling N = 1

• Observation of Fano-Resonances in Single-Wall Carbon Nanotubes
B. Babić and C. Schönenberger.
Phys. Rev. B, 70:195408, nov 2004. arXiv:0406571
[Abstract]

We have explored the low-temperature linear and nonlinear electrical conductance G of metallic carbon nanotubes (CNT’s), which were grown by the chemical-vapor deposition method. The high transparency of the contacts allows to study these two-terminal devices in the high conductance regime. We observe the expected four-fold shell pattern together with Kondo physics at intermediate transparency Gless than or similar to2e(2)/h and a transition to the open regime in which the maximum conductance is doubled and bound by G(max)=4e(2)/h. In the high-G regime, at the transition from a quantum dot to a weak link, the CNT levels are strongly broadened. Nonetheless, sharp resonances appear superimposed on the background which varies slowly with gate voltage. The resonances are identified by their lineshape as Fano resonances. The origin of Fano resonances is discussed along the modeling.

• Conductance properties of nanotubes coupled to superconducting leads: signatures of Andreev states dynamics
E. Vecino, M. R. Buitelaar, A. Martı́n-Rodero, C. Schönenberger, and Levy A. Yeyati.
Solid-State Communications 131, 625 (2004), 131:625-630, sep 2004. arXiv:0406240
[Abstract]

We present a combined experimental and theoretical analysis of the low bias conductance properties of carbon nanotubes coupled to superconducting leads. In the Kondo regime, the conductance exhibits a zero bias peak which can be several times larger than the unitary limit in the normal case. This zero bias peak can be understood by analyzing the dynamics of the subgap Andreev states under an applied bias voltage. It is shown that the existence of a linear regime is linked to the presence of a finite relaxation rate within the system. The theory provides a good fitting of the experimental results.

• Quantum dot coupled to a normal and a superconducting lead
M. R. Gräber, T. Nussbaumer, W. Belzig, and C. Schönenberger.
Nanotechnology, 15:S479-S482, may 2004.
[Abstract]

We report on electrical transport measurements in a carbon nanotube quantum dot coupled to a normal and a superconducting lead. Depending on the ratio of Kondo temperature T-K and superconducting gap Delta, the zero bias conductance resonance either is split into two side-peaks or persists. We also compare our data with a simple model of a resonant level-superconductor interface.

### 2003

• Sensitivity of single multiwalled carbon nanotubes to the environment
M. Krüger, I. Widmer, T. Nussbaumer, M. Buitelaar, and C. Schönenberger.
New Journal of Physics, 5:138.1-138.11, oct 2003.
[Abstract]

We report on electrical resistance measurements of single multiwalled carbon nanotubes (MWNTs) in different environments (ambient air, H2, O2 and the electrolytes LiClO4, KCl, KMnO4 and H3PO3). The gate dependence is studied using back-gating, electrochemical gating and gates evaporated directly onto the nanotubes (NTs). MWNTs at room temperature are p-doped. Upon changing the environment a change of the doping state of the MWNTs is inferred from the linear resistance. The effect of the environment on the contacts is negligible in our experiments. The p-doping is proposed to originate from the specific adsorption of an oriented dipole layer of water on the nanotube, which is affected by the kind of ions.

• Intrinsic thermal vibrations of suspended doubly clamped singe-wall carbon nanotubes
B. Babić, J. Furer, S. Sahoo, S. Farhangfar, and C. Schönenberger.
Nano Letters, 3:1577, sep 2003.
[Abstract]

We report the observation of thermally driven mechanical vibrations of suspended doubly clamped carbon nanotubes, grown by chemical vapor deposition (CVD). Several experimental procedures are used to suspend carbon nanotubes. The vibration is observed as a blurring in images taken with a scanning electron microscope. The measured vibration amplitudes are compared with a model based on linear continuum mechanics.

• Multiple Andreev Reflections in a Carbon Nanotube Quantum Dot
M. R. Buitelaar, W. Belzig, T. Nussbaumer, B. Babić, B. Bruder, and C. Schönenberger.
Phys. Rev. Lett., 91:57005, Aug 2003. arXiv:0304233
[Abstract]

We report resonant multiple Andreev reflections in a multiwall carbon nanotube quantum dot coupled to superconducting leads. The position and magnitude of the subharmonic gap structure is found to depend strongly on the level positions of the single-electron states which are adjusted with a gate electrode. We discuss a theoretical model of the device and compare the calculated differential conductance with the experimental data.

• Quantum Shot Noise
C. Beenakker and C. Schönenberger.
Physics Today, 56(5):37-42, May 2003. arXiv:0605025
[Abstract]

Fluctuations in the flow of electrons can signal the transition from particlelike to wavelike behavior and signify the nature of charge transport in mesoscopic systems.

• Ambipolar field-effect transistor on as-grown single-wall carbon nanotube
B. Babić, M. Iqbal, and C. Schönenberger.
Nanotechnology, 14:327-331, jan 2003.
[Abstract]

We use a simultaneous flow of ethylene and hydrogen gases to grow single-wall carbon nanotubes by chemical vapour deposition. Strong coupling to the gate is inferred from transport measurements for both metallic and semiconducting tubes. At low temperatures, our samples act as single-electron transistors where the transport mechanism is mainly governed by Coulomb blockade. The measurements reveal very rich quantized energy level spectra spanning from the valence to the conduction band. The Coulomb diamonds have similar addition energies on both sides of the semiconducting gap. Signatures of the subband population have been observed at intermediate temperature.

### 2002

• Quantum Dot in the Kondo Regime Coupled to Superconductors
M. R. Buitelaar, T. Nussbaumer, and C. Schönenberger.
Phys. Rev. Lett., 89(25):256801, dec 2002. arXiv:0209048
[Abstract]

The Kondo effect and superconductivity are both prime examples of many-body phenomena. Here we report transport measurements on a carbon nanotube quantum dot coupled to superconducting leads that show a delicate interplay between both effects. We demonstrate that the superconductivity of the leads does not destroy the Kondo correlations on the quantum dot when the Kondo temperature, which varies for different single-electron states, exceeds the superconducting gap energy.

• Nanomechanics of Microtubules
A. Kis, S. Kasas, B. Babić, A. J. Kulik, G. A. D. Briggs, C. Schönenberger, S. Cataicas, and L. Forró.
Phys. Rev. Lett., 89(24):248101, Dec 2002.
[Abstract]

We have determined the mechanical anisotropy of a single microtubule by simultaneously measuring the Young’s and the shear moduli in vitro. This was achieved by elastically deforming the microtubule deposited on a substrate tailored by electron-beam lithography with a tip of an atomic force microscope. The shear modulus is 2 orders of magnitude lower than the Young’s, giving rise to a length-dependent flexural rigidity of microtubules. The temperature dependence of the microtubule’s bending stiffness in the (5-40) degrees C range shows a strong variation upon cooling coming from the increasing interaction between the protofilaments.

• Shot noise of series quantum point contacts intercalating chaotic cavities
S. Oberholzer, E. V. Sukhorukov, C. Strunk, and C. Schönenberger.
Phys. Rev. B, 66:233304, dec 2002. arXiv:0105403
[Abstract]

Shot noise of series quantum point contacts forming a sequence of cavities in a two-dimensional electron gas are studied theoretically and experimentally. Noise in such a structure originates from local scattering at the point contacts as well as from chaotic motion of the electrons in the cavities. We found that the measured shot noise is in reasonable agreement with our theoretical prediction taking the cavity noise into account

• The amplitude of non-equilibrium quantum interference in metallic mesoscopic systems
C. Terrier, D. Babić, C. Strunk, T. Nussbaumer, and C. Schönenberger.
Europhys. Lett., 59(3):437-443, aug 2002. arXiv:0103486
[Abstract]

We study the influence of a DC bias voltage V on quantum interference corrections to the measured differential conductance in metallic mesoscopic wires and rings. The amplitude of both universal conductance fluctuations (UCF) and Aharonov-Bohm effect (ABE) is enhanced several times for voltages larger than the Thouless energy. The enhancement persists even in the presence of inelastic electron-electron scattering up to V ~ 1 mV. For larger voltages electron-phonon collisions lead to the amplitude decaying as a power law for the UCF and exponentially for the ABE. We obtain good agreement of the experimental data with a model which takes into account the decrease of the electron phase-coherence length due to electron-electron and electron-phonon scattering.

• Vortex motion noise in micrometer-sized thin films of the amorphousNb0.7Ge0.3weak-pinning superconductor
D. Babić, T. Nussbaumer, C. Strunk, C. Schönenberger, and C. Sürgers.
Phys. Rev. B, 66:14537, Jul 2002.
[Abstract]

We report high-resolution measurements of voltage ~V! noise in the mixed state of micrometer-sized thin films of amorphous Nb0.7Ge0.3 , which is a good representative of weak-pinning superconductors. There is a remarkable difference between the noise below and above the irreversibility field Birr . Below Birr , in the presence of measurable pinning, the noise at small applied currents resembles shot noise, and in the regime of flux flow at larger currents decreases with increasing voltage due to a progressive ordering of the vortex motion. At magnetic fields B between Birr and the upper critical field Bc2 flux flow is present already at vanishingly small currents. In this regime the noise scales with (1-B/Bc2)^2V^2 and has a frequency (f) spectrum of 1/f type. We interpret this noise in terms of the properties of strongly driven depinned vortex systems at high vortex density.

• Orientation and Positioning of DNA molecules with an electric field technique
F. Dewarrat, M. Calame, and C. Schönenberger.
Single Mol., 3(4):189-193, Jul 2002.
[Abstract]

A controlled handling of single molecules is essential for the fabrication and the investigation of devices based on molecules. We present here the implementation of an electric field based method used to manipulate DNA molecules by means of lithographically patterned metallic electrodes. We optimized the geometry of the lithographic structures to favor a precise positioning of the molecules via dielectrophoresis. This process is combined with an orientation of the molecules parallel to the electric field lines due to their induced dipole moment. The relatively high polarizability of the DNA molecules in solution is essential to achieve these manipulations. We expect this method to be softer than the stretching of molecules using a receding meniscus. The visualization of the molecules was achieved using fluorescence microscopy.

• UHV compatible nanostructuring technique for mesoscopic hybrid devices: application to superconductor/ferromagnet Josephson contacts
T. Hoss, C. Strunk, C. Sürgers, and C.~Schönenberger.
Physica E, 14:341-345, may 2002.
[Abstract]

We report on an ultra-high vacuum (UHV) compatible method for fabricating devices of sub-micrometer size by virtue of a non-organic evaporation mask of high thermal and mechanical stability. As an application we describe the superconducting properties of mesoscopic superconductor/normal-metal and superconductor/ferromagnet/superconductor hybrid structures. In particular, we report on the observation of the DC-Josephson effect in Nb/Cu/Co/Cu/Nb structures prepared in UHV. The Josephson coupling between the two superconductors through the very thin (5nm) magnetic and metallic weak link is confirmed by the magnetic field dependence of the critical current Ic, which displays a Fraunhofer-like interference pattern.

• Multi-wall carbon nanotubes as quantum dots
M. R. Buitelaar, A. Bachtold, T. Nussbaumer, M. Iqbal, and C. Schönenberger.
Phys. Rev. Lett., 88(15):156801, apr 2002. arXiv:0110074
[Abstract]

We have measured the differential conductance dI/dV of individual multi-wall carbon nanotubes (MWNT) of different lengths. A cross-over from wire-like (long tubes) to dot-like (short tubes) behavior is observed. dI/dV is dominated by random conductance fluctuations (UCF) in long MWNT devices (L=2…7 μm), while Coulomb blockade and energy level quantization are observed in short ones (L=300 nm). The electron levels of short MWNT dots are nearly four-fold degenerate (including spin) and their evolution in magnetic field (Zeeman splitting) agrees with a g-factor of 2. In zero magnetic field the sequential filling of states evolves with spin S according to S=0 -> 1/2 -> 0… In addition, a Kondo enhancement of the conductance is observed when the number of electrons on the tube is odd.

• Crossover between classical and quantum shot noise in chaotic cavities
S. Oberholzer, E. V. Sukhorukov, and C. Schönenberger.
Nature, 415:765-767, feb 2002.
[Abstract]

The discreteness of charge in units of e led Schottky in 1918 to predict that the electrical current in a vacuum tube fluctuates even if all spurious noise sources are eliminated carefully1. This phenomenon is now widely known as shot noise. In recent years, shot noise in mesoscopic conductors, where charge motion is quantum-coherent over distances comparable to the system size, has been studied extensively2, 3, 4, 5. In those experiments, charge does not propagate as an isolated entity through free space, as for vacuum tubes, but is part of a degenerate and quantum-coherent Fermi sea of charges. It has been predicted that shot noise in mesoscopic conductors can disappear altogether when the system is tuned to a regime where electron motion becomes classically chaotic6. Here we experimentally verify this prediction by using chaotic cavities where the time that electrons dwell inside can be tuned7. Shot noise is present for large dwell times, where the electron motion through the cavity is ‘smeared’ by quantum scattering, and it disappears for short dwell times, when the motion becomes classically deterministic

• Fabrication and superconducting properties of nanostructured SFS contacts
C. Sürgers, T. Hoss, C. Strunk, and C. Schönenberger.
Journal of Magnetism and Magnetic Materials, 240:598-600, feb 2002.
[Abstract]

Superconducting hybrid structures of submicrometer size utilizing high-melting transition metals such as Nb or Ta can be fabricated in ultra-high vacuum by means of a non-organic evaporation mask (Si3N4) of high thermal and mechanical stability. We report on the magnetic and superconducting properties of mesoscopic superconductor/ferromagnet/superconductor (SFS) junctions realized in a Nb/Cu/Co/Cu/Nb multilayer (ML). Below the superconducting transition temperature, the magnetic hysteresis loop shows a contribution from the strongly pinned magnetic flux of the superconducting Nb layers. Electrical transport measurements perpendicular to the layered structure clearly demonstrate a Josephson coupling between the Nb layers through the 5-nm thick ferromagnetic Co film.

### 2001

• Shot Noise in Schottky’s Vacuum Tube is Classical
C. Schönenberger, S. Oberholzer, E. V. Sukhorukov, and H. Grabert.
cond-mat/0112504, pages 1-5, dec 2001. arXiv:0112504
[Abstract]

In these notes we discuss the origin of shot noise (‘Schroteffekt’) of vacuum tubes in detail. It will be shown that shot noise observed in vacuum tubes and first described by W. Schottky in 1918 is a purely classical phenomenon. This is in pronounced contrast to shot noise investigated in mesoscopic conductors which is due to quantum mechanical diffraction of electron waves.

• Suppression of tunneling into multi-walled carbon nanotubes
M. Bachtold, M. de Jonge, K. Grove-Rasmussen, P. L. McEuen, M. Buitelaar, and C.~Schönenberger.
Phys. Rev. Lett., 87(16):166801, oct 2001. arXiv:0012262
[Abstract]

We have studied tunneling of electrons into multi-wall carbon nanotubes. Nanotube/electrode interfaces with low transparency as well as nanotube/nanotube junctions created with atomic force microscope manipulation have been used. The tunneling conductance goes to zero as the temperature and bias are reduced, and the functional form is consistent with a power law suppression of tunneling as a function of energy. The exponent depends upon sample geometry. The relationship between these results and theories for tunneling into ballistic and disordered metals is discussed.

• Comment on Magnetoresistance and differential conductance in multiwalled carbon nanotubes’
C. Schönenberger and A. Bachtold.
Phys. Rev. B, 64:157401, sep 2001.
[Abstract]

Jeong-O Lee et al. [Phy. Rev. B, 61, R16 362 (2000)] reported magnetoresistance and differential conductance measurements of multiwalled carbon nanotubes. The observed aperiodic conductance fluctuations and the negative magnetoresistance was interpreted to originate exclusively from changes in the density of states at the Fermi energy. We show that this interpretation is questionable and not supported by their measurements.

• Shot Noise by Quantum Scattering in Chaotic Cavities
S.~Oberholzer, E.~V.~Sukhorukov, C.~Strunk, C.~Schönenberger, T.~Heinzel, and M.~Holland.
Phys. Rev. Lett., 86(10):2114-2117, mar 2001. arXiv:0009087
[Abstract]

We have experimentally studied shot noise of chaotic cavities defined by two quantum point contacts in series. The cavity noise is determined as (1/4)2e/I/ in agreement with theory and can be well distinguished from other contributions to noise generated at the contacts. Subsequently, we have found that cavity noise decreases if one of the contacts is further opened and reaches nearly zero for a highly asymmetric cavity. Heating inside the cavity due to electron-electron interaction can slightly enhance the noise of large cavities and is also discussed quantitatively.

• Electrochemical carbon nanotube field-effect transistor
M. Krüger, M. Buitelaar, T. Nussbaumer, C.~Schönenberger, and L. Forró.
App. Phys. Lett., 78(9):1291-1293, feb 2001. arXiv:0009171
[Abstract]

We explore the electric-field effect of carbon nanotubes (NTs) in electrolytes. Due to the large gate capacitance, Fermi energy (EF)(EF) shifts of order ±1 V can be induced, enabling to tune NTs from p to n-type. Consequently, large resistance changes are measured. At zero gate voltage, the NTs are hole-doped in air with |EF|≈0.3–0.5 eV,|EF|≈0.3–0.5 eV, corresponding to a doping level of ≈10 13 cm−2.≈10 13 cm−2. Hole-doping increases in the electrolyte.

• Carbon nanotubes, materials for the future
L. Forró and C.~Schönenberger.
Europhysics news, 32(3):86-90, May/June 2001.

### 2000

• Interference and interactions in multiwall nanotubes
C.~Strunk, A.~Bachtold, T.~Nussbaumer, and C.~Schönenberger.
Physica B, 280(1-4):384-385, sep 2000.
[Abstract]

We report equilibrium electric resistance R and tunneling spectroscopy (dI/dV)measurements obtained on single multi-wall nanotubes contacted by four metallic Au fingers from above. At low temperature quantum interference phenomena dominate the magnetoresistance. The phase-coherence (lφ)and elastic-scattering lengths (le)are deduced. Because le is of order of the circumference of the nanotubes, transport is quasi-ballistic. This result is supported by a dI/dV spectrum which is in good agreement with the density of states (DOS) due to the one-dimensional subbands expected for a perfect single-wall tube. As a function of temperature T the resistance increases on decreasing T and saturates at ≈1–10 Kfor all measured nanotubes. R(T) cannot be related to the energy-dependent DOS of graphene but is mainly caused by interaction and interference effects. On a relatively small voltage scale of the order ≈10 meV, a pseudogap is observed in dI/dV which agrees with Luttinger-liquid theories for nanotubes. Because we have used quantum diffusion based on Fermi-liquid as well as Luttinger-liquid theory in trying to understand our results, a large fraction of this paper is devoted to a careful discussion of all our results.

• Multiple Andreev reflection and giant excess noise in diffusive superconductor/normal-metal/superconductor junctions
T.~Hoss, C.~Strunk, T.~Nussbaumer, R.~Huber, U.~Staufer, and C.~Schönenberger.
Phys. Rev. B, 62(6):4079-4085, aug 2000. arXiv:9901129
[Abstract]

We have studied superconductor/normal metal/superconductor (SNS) junctions consisting of short Au or Cu wires between Nb or Al banks. The Nb based junctions display inherent electron heating effects induced by the high thermal resistance of the NS boundaries. The Al based junctions show in addition subharmonic gap structures in the differential conductance dI/dV and a pronounced peak in the excess noise at very low voltages V. We suggest that the noise peak is caused by fluctuations of the supercurrent at the onset of Josephson coupling between the superconducting banks. At intermediate temperatures where the supercurrent is suppressed a noise contribution ~1/V remains, which may be interpreted as shot noise originating from large multiple charges.

• Physics of Multiwall-Carbon Nanotubes
C. Schönenberger and L. Forró.
Physics World, 13(6):37-41, June 2000.

• The Hanbury Brown and Twiss experiment with fermions
S. Oberholzer, M. Henny, C. Strunk, C. Schönenberger, T. Heinzel, K. Ensslin, and M. Holland.
Physica E, 6:314-317, feb 2000.
[Abstract]

We realized an equivalent Hanbury Brown and Twiss experiment for a beam of electrons in a two-dimensional electron gas in the quantum Hall regime. A metallic split gate serves as a tunable beam splitter which is used to partition the incident beam into transmitted and reflected partial beams. The current fluctuations in the reflected and transmitted beam are fully anticorrelated demonstrating that fermions tend to exclude each other (anti-bunching). If the occupation probability of the incident beam is lowered by an additional gate, the anticorrelation is reduced and disappears in the classical limit of a highly diluted beam.

• Colloidal Dispersions of Gold Rods:  Synthesis and Optical Properties
M. I. van der Zande, M. R. Böhmer, L. G. J. Fokkink, and C. Schönenberger.
Langmuir, 16:451-458, 2000.

### 1999

• Interference and Interaction in multi-wall carbon nanotubes
C. Schönenberger, A. Bachtold, C. Strunk, J. -P. Salvetat, and L. Forró.
Appl. Phys. A, 69:283-295, sep 1999.
[Abstract]

We report equilibrium electric resistance R and tunneling spectroscopy (dI/dV)measurements obtained on single multi-wall nanotubes contacted by four metallic Au fingers from above. At low temperature quantum interference phenomena dominate the magnetoresistance. The phase-coherence (lφ)and elastic-scattering lengths (le)are deduced. Because le is of order of the circumference of the nanotubes, transport is quasi-ballistic. This result is supported by a dI/dV spectrum which is in good agreement with the density of states (DOS) due to the one-dimensional subbands expected for a perfect single-wall tube. As a function of temperature T the resistance increases on decreasing T and saturates at ≈1–10 Kfor all measured nanotubes. R(T) cannot be related to the energy-dependent DOS of graphene but is mainly caused by interaction and interference effects. On a relatively small voltage scale of the order ≈10 meV, a pseudogap is observed in dI/dV which agrees with Luttinger-liquid theories for nanotubes. Because we have used quantum diffusion based on Fermi-liquid as well as Luttinger-liquid theory in trying to understand our results, a large fraction of this paper is devoted to a careful discussion of all our results.

• Nonorganic evaporation mask for superconducting nanodevices
T. Hoss, C. Strunk, and C. Schönenberger.
Microelectronic Engineering, 46:149, may 1999.
[Abstract]

We describe a novel technique to produce submicron thin film structures of high melting superconducting materials (Nb). The method is based on a nonorganic evaporation mask (Si3N4) to avoide any outgassing of the mask material during the metal deposition which would deteriorate the superconducting properties of the Nb. The mask has a large offset from the substrate so that clean interfaces of different materials (e.g. normal metal/superconductor (NS) can be achieved by angle evaporation in one single process step. By this means we have prepared narrow Nb wires with high transition temperature and NS structures with high quality interfaces.

• Electrical conduction through DNA molecules
H. -W. Fink and C. Schönenberger.
Nature, 398:407, apr 1999.
[Abstract]

The question of whether DNA is able to transport electrons has attracted much interest, particularly as this ability may play a role as a repair mechanism after radiation damage to the DNA helix1. Experiments addressing DNA conductivity have involved a large number of DNA strands doped with intercalated donor and acceptor molecules, and the conductivity has been assessed from electron transfer rates as a function of the distance between the donor and acceptor sites2,3. But the experimental results remain contradictory, as do theoretical predictions4. Here we report direct measurements of electrical current as a function of the potential applied across a few DNA molecules associated into single ropes at least 600 nm long, which indicate efficient conduction through the ropes. We find that the resistivity values derived from these measurements are comparable to those of conducting polymers, and indicate that DNA transports electrical current as efficiently as a good semiconductor. This property, and the fact that DNA molecules of specific composition ranging in length from just a few nucleotides to chains several tens of micrometres long can be routinely prepared, makes DNA ideally suited for the construction of mesoscopic electronic devices.

• Aharonov-Bohm Oscillations in Carbon Nanotubes
A. Bachtold, C. Strunk, J. -P. Salvetat, J. -M. Bonard, L. Forró, T. Nussbaumer, and C. Schönenberger.
Nature, 397:673, feb 1999.
[Abstract]

When electrons pass through a cylindrical electrical conductor aligned in a magnetic field, their wave-like nature manifests itself as a periodic oscillation in the electrical resistance as a function of the enclosed magnetic flux1. This phenomenon reflects the dependence of the phase of the electron wave on the magnetic field, known as the Aharonov–Bohm effect2, which causes a phase difference, and hence interference, between partial waves encircling the conductor in opposite directions. Such oscillations have been observed in micrometre-sized thin-walled metallic cylinders3, 4, 5 and lithographically fabricated rings6, 7, 8. Carbon nanotubes9, 10 are composed of individual graphene sheets rolled into seamless hollow cylinders with diameters ranging from 1 nm to about 20 nm. They are able to act as conducting molecular wires11, 12, 13, 14, 15, 16, 17, 18, making them ideally suited for the investigation of quantum interference at the single-molecule level caused by the Aharonov–Bohm effect. Here we report magnetoresistance measurements on individual multi-walled nanotubes, which display pronounced resistance oscillations as a function of magnetic flux.We find that the oscillations are in good agreement with theoretical predictions for the Aharonov–Bohm effect in a hollow conductor with a diameter equal to that of the outermost shell of the nanotubes. In some nanotubes we also observe shorter-period oscillations, which might result from anisotropic electron currents caused by defects in the nanotube lattice.

• 1/3-shot-noise suppression in diffusive nanowires
M. Henny, S. Oberholzer, C. Strunk, and C. Schönenberger.
Phys. Rev. B., 59:2871-2880, jan 1999.
[Abstract]

We report low-temperature shot noise measurements of short diffusive Au wires attached to electron reservoirs of varying sizes. The measured noise suppression factor compared to the classical noise value 2e|I| strongly depends on the electric heat conductance of the reservoirs. For small reservoirs injection of hot electrons increases the measured noise and hence the suppression factor. The universal 1/3-suppression factor can only asymptotically be reached for macroscopically large and thick electron reservoirs. A heating model based on the Wiedemann-Franz law is used to explain this effect.

• The Fermionic Hanbury-Brown & Twiss Experiment
M. Henny, S. Oberholzer, C. Strunk, T. Heinzel, K. Ensslin, M. Holland, and C. Schönenberger.
Science, 284:296, 1999.
[Abstract]

A Hanbury Brown and Twiss experiment for a beam of electrons has been realized in a two-dimensional electron gas in the quantum Hall regime. A metallic split gate serves as a tunable beam splitter to partition the incident beam into transmitted and reflected partial beams. In the nonequilibrium case the fluctuations in the partial beams are shown to be fully anticorrelated, demonstrating that fermions exclude each other. In equilibrium, the cross-correlation of current fluctuations at two different contacts is also found to be negative and nonzero, provided that a direct transmission exists between the contacts.

### 1998

• Size Dependent Thermopower in Mesoscopic AuFe Wires
C. Strunk, M. Henny, C. Schönenberger, G. Neuttiens, and Van C. Haesendonck.
Phys. Rev. Lett., 81:2982-2985, oct 1998.
[Abstract]

We have combined electron heating experiments and noise thermometry to perform quantitative measurements of the thermopower in mesoscopic samples. This new measuring technique allows us to detect finite size effects in the thermopower of narrow AuFe wires with an Fe concentration ranging from 50 to 3000 ppm. The size effects emerge when reducing the width of the wires below ~300 nm and may be related to a spin-orbit induced magnetic anisotropy close to the wire surface.

• Contacting Carbon-Nanotubes selectively with Low-Ohmic Contacts for Four-Probe Electric Measurements
A. Bachtold, J. -P. Salvetat, J. -M. Bonard, M. Henny, C. Terrier, C. Strunk, L. Forró, and C. Schönenberger.
Appl. Phys. Lett., 73:274-276, jul 1998.
[Abstract]

Contact resistances of multiwalled nanotubes deposited on gold contact fingers are very large. We show that the contact resistances decrease by orders of magnitudes when the contact areas are selectively exposed to the electron beam in a scanning electron microscope. The focused electron beam enables the selection of one particular nanotube for electrical measurement in a four-terminal configuration, even if a loose network of nanotubes is deposited on the gold electrodes. For all measured nanotubes, resistance values lie in a narrow range of 0.35–2.6 kΩ at room temperature.

• Contacting single template synthesized nanowires for electric measurements
A. Bachtold, C. Terrier, M. Krüger, M. Henny, T. Hoss, C. Strunk, R. Huber, H. Birk, U. Staufer, and C. Schönenberger.
Microelectronic Engineering, 41/42:571-574, mar 1998.
[Abstract]

With template synthesis nanowires with diameters as small as 5 nm can be fabricated using electrochemical plating in nanopores. In this work contacts are fabricated enabling electrical measurements on one nanowire. A combination of chemical methods and e-beam lithography is used. The successful contacting is demonstrated for the case of Ni wires.

### 1997

• Template Synthesis of Nanowires in Porous Polycarbonate Membranes:  Electrochemistry and Morphology
C. Schönenberger, B. M. I. van der Zande, L. G. J. Fokkink, M. Henny, C. Schmid, M. Krüger, A. Bachtold, R. Huber, H. Birk, and U. Staufer.
J. Phys. Chem. B, 101:5497-5505, jul 1997.
[Abstract]

The potentiostatic electrochemical template synthesis of nanowires (Ni, Co, Cu, Au, and polyporrole) in polycarbonate track-etched membranes with nominal pore diameters dN between 10 and 200 nm is studied. Along the wire the cross section is found to vary:  the wire diameter, which is argued to directly reflect the pore diameter, is observed (for all deposits) to be substantially larger in the middle than at both ends. Therefore, the pores are not cylindrical with constant cross-section, in general, but appear to be “cigarlike”. Inside the membrane, the pores are wider by up to a factor 3. Comparing the potentiostatically measured current-time characteristics obtained during wire growth for different pore dimensions, a pore-size dependence of the diffusion coefficient D for the metal ions is found:  D = 2.5, 1.5, and 0.7 × 10-6 cm2/s for dN = 80, 30, and 10 nm, respectively.

• Electron heating effects in diffusive metal wires
M. Henny, H. Birk, R. Huber, C. Strunk, A. Bachtold, M. Krüger, and C. Schönenberger.
Appl. Phys. Lett., 71:773-775, jun 1997.
[Abstract]

We have investigated the electron heating in metallic diffusive wires of varying length at liquid-helium temperature by measuring the electric noise. The local increase of the electron temperature can be essential already for small currents and is well described by a heat-diffusion equation for the electrons. Depending on the electron thermal conductance and the electron–phonon coupling in the wire, different length regimes are identified. The quantitative knowledge of the electron temperature is important for analysis of nonequilibrium effects involving current heating in mesoscopic wires.

• Aqueous Gold Sols of Rod-Shaped Particles
M. I. van der Zande, Marcel R. Böhmer, Lambertus G. J. Fokkink, and C. Schönenberger.
J. Phys. Chem. B, 101:852-854, feb 1997.
[Abstract]

Aqueous dispersions of rodlike gold particles are obtained by electrodeposition in nanopores of anodized alumina attached to a conductive support followed by dissolution of the alumina and stabilization of the rods with poly(vinylpyrrolydon). The obtained sol of monodisperse gold rods is examined by electron microscopy and visible (VIS) and near-infrared (NIR) spectroscopy. In the VIS/NIR absorption spectra two absorption maxima are present. With increasing aspect ratio, the maximum around 520 nm shifts to shorter wavelength, while the other maximum shifts into the near-infrared regime, which is in agreement with theoretical predictions.

• Microscope images individual charges
Christian Schönenberger.
Physics World, 10:25-27, 1997.
[Abstract]

Although microscopy – the technology of imaging objects on the micrometre scale and smaller – is an old discipline, the development of new instruments and techniques has led to impressive achievements during the last few decades. Today’s electron microscopes can image the crystallographic structure of thin films with atomic resolution. This achievement could have been anticipated long ago, since it is based on the short wavelength of the electrons. In contrast, the versatility and high resolution of scanning probe microscopes came as a surprise. One recent example is the development at Bell Laboratories in the US of a device that can image electric charges on surfaces with a sensitivity better than that of a single electron (M J Yoo et al. 1997 Science 276 579).

### 1996

• Nanolithography on hydrogen-terminated silicon by scanning-probe microscopy
Christian Schönenberger and Niels Kramer.
Microelectronic Engineering, 32:203-217, sep 1996.
[Abstract]

Scanning-probe microscopes (SPM), i.e. the scanning-tunneling and force microscopes, can be used to locally oxidize hydrogen-terminated silicon and hydrogenated amorphous silicon. Because of its reliability and potential for pattern transfer, this lithography process has found great attention and has become a prototype process for SPM nanolithography. The local oxidization can be performed in ambient or ultra-high vacuum (UHV), and it is initiated by strong electric fields, electron impact, or by short-wavelength light. In this article, the progress of this subfield of nanolithography is reviewed. Emphasis will be on the process conducted in humid environments were a fairly solid understanding is emerging. For completeness, important experiments performed in UHV will be discussed briefly. Finally, recent applications of this process technique to the fabrication of electronic devices will be presented.

• Preamplifier for electric current noise measurements at low temperatures
H. Birk, K. Oostveen, and C. Schönenberger.
Rev. Sci. Instr., 67:2977-2980, apr 1996.
[Abstract]

We have developed a current preamplifier that operates in a liquid-helium bath cryostat. It has been optimized for the measurement of dynamical electric-current fluctuations (noise) of high-impedance sources R>100 MΩ. A bandwidth of up to 400 kHz has been achieved by effectively minimizing the capacitance of the input transistor with a dynamical feedback. The amplifier measures current noise in a scanning tunneling microscope (STM). It enables the measurement of shot noise for currents as low as 30 pA (sampling rate 5 s) for a high-impedance source with a resistance of R>1 GΩ, a value typical for tunneling resistances in STM.

### 1995

• Decapitation of tungsten field emitter tips during sputter sharpening
C. Schiller, A. A. Koomans, T. L. van Rooy, C. Schönenberger, and H. B. Elswijk.
Surface Science, 339:L925-L930, oct 1995.
[Abstract]

The sharpening of tungsten tips by sputtering with neon ions shows a surprising phenomenon when the tip radius is smaller than about 20 nm: the gas ions produce a neck in the tip which becomes thinner until it breaks due to the electrostatically induced stress. This decapitation phenomenon limits the attainable tip sharpness to a radius of about 4 nm. The process can repeat in a regular, oscillating manner and is probably at the origin of the unreliability often mentioned in connection with this sharpening method. The effect can be used to determine experimentally the theoretical shear stress of refractory metals.

• Resistless high resolution optical lithography on silicon
N. Kramer, M. Niesten, and C. Schönenberger.
Appl. Phys. Lett., 67:2989-2991, sep 1995.
[Abstract]

In this letter, we report on the high resolution patterning of a silicon surface without using a resist layer. A hydrogen passivated silicon surface is chemically modified by illumination with ultraviolet light (UV, λ=350.7 nm) in air. Auger electron spectroscopy (AES) revealed that silicon oxide was formed at the illuminated areas. A light interference pattern was made on the silicon surface by two UV laser beams, oxidation occurred only at the maximum intensity, but not at the minimum. In this way oxide lines were fabricated with a width below 200 nm on a 500 nm period. The oxide lines were used as a wet etch mask to etch more than 25 nm into Si(110) without affecting the oxide. The advantage of this technique is that it is a very simple process which allows the high resolution patterning over large areas of silicon without using a resist.

• Shot-Noise Suppression in the Single-Electron Tunneling Regime
H. Birk, M. J. M. de Jong, and C. Schönenberger.
Phys. Rev. Lett., 75:1610-1613, aug 1995.
[Abstract]

Electrical current fluctuations through tunnel junctions are studied with a scanning-tunneling microscope. For single-tunnel junctions classical Poisson shot noise is observed, indicative for uncorrelated tunneling of electrons. For double-barrier tunnel junctions, formed by a nanoparticle between tip and surface, the shot noise is observed to be suppressed below the Poisson value. For strongly asymmetric junctions, where a Coulomb staircase is observed in the current-voltage characteristic, the shot-noise suppression is periodic in the applied voltage. This originates from correlations in the transfer of electrons imposed by single-electron charging effects.

• Giant magnetoresistance of electrodeposited Co/Cu multilayers
S. K. J. Lenczowski, C. Schönenberger, M. A. M. Gijs, and W. J. M. de Jonge.
J. Magn. Magn. Mater., 148:455-465, jul 1995.
[Abstract]

We report on the structural and electrical characterization of electrodeposited Co/Cu multilayers grown in a single electrolyte based on CoSO4 and CuSO4. A high degree of crystallographic orientation and superlattice coherence is found in the growth on (100)- and (111)-oriented substrates. The magnetoresistance (MR), measured in the current-in-plane configuration at room temperature, is dominated by the giant MR effect for Cu-layer thicknesses dCu > 3nm and by the anisotropic MR effect for dCu < 2.5nm. A maximum of 14% is measured for dCu ≈ 4 nm. No evidence for antiferromagnetic coupling is found. Instead, the giant MR gradually diminishes with decreasing dCu < 4 nm which is attributed to ferromagnetic coupling due to magnetic pinholes. The influence of the Cu2+-ion concentration, the addition of levelling agents, and the Co-and Cu-layer thicknesses on the structure and magnetoresistance is systematically investigated. Especially the use of levelling agents has a catastrophic effect on the structural quality of the multilayers and on the magnitude of the MR.

• Fabrication of large arrays of metallic nanowires on V-grooved substrates
J. Jorritsma, M. A. M. Gijs, C. Schönenberger, and J. G. H. Stienen.
Appl. Phys. Lett., 67:1489-1491, jun 1995.
[Abstract]

Large arrays of Au nanowires down to 50 nm in width are fabricated on V-grooved InP substrates. Holographic laser interference exposure of photoresist and anisotropic etching are used to pattern the surface of InP(001) substrates into V-shaped grooves with a 200 nm period. Next, the patterned substrates are covered with a thin Au film, which is subsequently structured into nanowires using a well controlled wet etching process. Initial characterization confirms that the wires are electrically continuous.

• Fabrication of metallic nanowires with a scanning tunneling microscope
N. Kramer, H. Birk, J. Jorritsma, and C. Schönenberger.
Appl. Phys. Lett., 66:1325-1327, jun 1995.
[Abstract]

A procedure to pattern thin metal films on a nanometer scale with a scanning tunneling microscope (STM) operating in air is reported. A 30 nm film of hydrogenated amorphous silicon (a-Si:H) is deposited on a 10 nm film of TaIr. Applying a negative voltage between the STM tip and the a-Si:H film causes the local oxidation of a-Si:H. The oxide which is formed is used as a mask to wet etch the not-oxidized a-Si:H and subsequently, the remaining pattern is transferred into the metal film by Ar ion milling. Metal wires as narrow as 40 nm have been fabricated. Since a-Si:H can be deposited in very thin layers on almost any substrate, the presented procedure can be applied to structure all kind of thin films on a nanometer scale.

• Nanometer Lithography on Silicon and Hydrogenated Amorphous Silicon with Low Energy Electrons
N. Kramer, J. Jorritsma, H. Birk, and C. Schönenberger.
J. Vac. Sci. Technol. B, 13:805, mar 1995.
[Abstract]

The oxidation of a hydrogen terminated Si surface can locally be induced with a scanning tunnelling microscope (STM) operating in air or with a beam of free electrons in a controlled oxygen environment. The oxidation mechanism of both processes was studied and compared. The oxidation with the STM in air depends strongly on the applied tip‐substrate voltage and writing speed, but is not proportional to the tunnelling current. This is in contrast to the process with a beam of free electrons. The thickness of the electron beam induced oxide is studied as a function of electron energy, electron dose, and oxygen pressure. Oxide thicknesses of 0.5–3 nm are measured using Auger spectroscopy. The initial step of the oxidation process is the electron beam induced removal of hydrogen from the surface. The electron dose requirement for this step was determined as a function of electron energy. The dose is found to be minimal for 100 eV electrons, and is ≊4 mC/cm2. Oxide lines made with the STM on Si(110) were used as a mask to wet etch the pattern into the Si(110). With tetramethyl ammonium hydroxide, a selective anisotropic etch liquid, trenches with a width of 35 nm and a depth of 300 nm were made. We show that it is also possible to locally oxidize hydrogenated amorphous silicon (a‐Si:H) and use the oxide as an etching mask. Hydrogenated amorphous silicon has the advantage that it can be deposited in very thin layers on almost any substrate and therefore has great potential as STM and electron‐beam resist.

• Domain Structure of Self-Assembled Alkanethiol Monolayers on Gold
C. Schönenberger, J. Jorritsma, J. A. M. Sondag-Huethorst, and L. G. J. Fokkink.
J. Phys. Chem., 99:3259-3271, mar 1995.

• Nanometer Lithography on Silicon and Hydrogenated Amorphous Silicon with Low Energy Electrons
N. Kramer, J. Jorritsma, H. Birk, and C. Schönenberger.
Microelectronic Engineering, 27:47-50, feb 1995.
[Abstract]

We report the local oxidation of hydrogen terminated silicon (Si) surfaces induced with the scanning-tunneling microscope (STM) operating in air and by a beam of free low-energy electrons. With STM, oxide lines were written in Si(100) and Si(110) and transferred into the substrate by wet etching. In case of Si(110) trenches with a width as small as 35 nm and a depth of 300 nm were made. The same process has also successfully been applied to the patterning of hydrogenated amorphous silicon (a-Si:H) thin films. We demonstrate the fabrication of metallic ‘nanowires’ using a-Si:H as resist layer. With regard to the process of oxidation, it is found that the oxide written with STM is apparently not proportional to the electron current, in contrast to results obtained with a beam of free electrons in an oxygen gas-environment. The dose needed to remove the hydrogen was determined as a function of electron energy. This dose is minimal for 100 eV electrons amounting to 4 mC/cm2.

### 1994

• Formation of Holes in Alkanethiol Monolayers on Gold
J. A. M. Sondag-Huethorst, C. Schönenberger, and L. G. J. Fokkink.
J. Phys. Chem., 98:6826-6834, jul 1994.

• What are the `holes’ in self-assembled monolayers of alkanethiols on Au
C. Schönenberger, J. A. M. Sondag-Huethorst, J. Jorritsma, and L. G. J. Fokkink.
Langmuir, 10:611-614, mar 1994.

### 1993

• Single-electron tunneling in double-barrier junctions by scanning tunneling microscopy
C. Schönenberger, H. van Houten, J. M. Kerkhof, and H. C. Donkersloot.
Appl. Surf. Sci., 67:222-227, 1993.

• Polarization charge relaxation and the Coulomb staircase in ultra-small double-barrier tunnel junctions
C. Schönenberger, C. W. J. Beenakker, and H. van Houten.
Physica B, 189:218-224, 1993.

### 1992

• Single-Electron Tunnelling Observed At Room Temperature by Scanning-Tunnelling Microscopy
C. Schönenberger, H. van Houten, and H. C. Donkersloot.
Europhys. Lett., 20:249-254, oct 1992.
[Abstract]

Ultrasmall (lesssim 5 nm in lateral diameter) double-barrier tunnel junctions have been realized using a scanning tunnelling microscope, and an optimized metal particle-oxide-metallic substrate system. Three electrical transport effects, all in good agreement with the semi-classical theory of single-electron tunnelling, have been found at room temperature: the Coulomb gap, the Coulomb staircase and zero-bias conductance oscillations as a function of tip-particle distance.

• Characterization of titanium nanoscopic wire by STM and SFM
C. Joachim, B. Rousset, C. Schönenberger, A. Kerrien, E. Druet, and J. Chevalier.
Nanotechnology, 2(96), jun 1992.
[Abstract]

STM and SFM near-field scanning microscopies are compared with the standard SEM microscopy for their ability to characterize an 80 nm wide titanium wire. The dimensions measured with SEM, STM and SFM are comparable. However, without a metallization of the structure, SFM provides better information on the relief of this nanowire. Moreover, it is shown how an electrostatic SFM can provide direct information on the metallic character of the nanowire with the same resolution as the topography.

• Charge flow during metal-insulator contact
C. Schönenberger.
Phys. Rev. B, 45:3861-3864, 1992.

• Single-electron tunneling up to room temperature
C. Schönenberger, H. van Houten, H. C. Donkersloot, A. M. T. van der Putten, and L. G. L. Fokkink.
Physica Scripta, T45:289-291, 1992.

### 1991

• Magnetic force microscopy and its applications to longitudional thin films
C. Schönenberger, S. F. Alvarado, S. E. Lambert, and I. L. Sanders.
J. Mag. Mag. Mat., 93:123-127, feb 1991.
[Abstract]

The interpretation of magnetic force microscopy (MFM) images requires knowledge of the magnetic structure within the tip. The etched polycrystalline iron, cobalt and nickel wires investigated all have their apex domain oriented along the tip axis. The image formation is found to be determined by an effective domain length L. For magnetization patterns of scales large compared to L, the point dipole approximation is applicable, provided that the tip apex can be treated as an isolated domain. While this is observed to be the case for cobalt tips, it does not apply to the most common tip materials, i.e. nickel and iron. The resolving power of a tip is given by its tip radius. At sufficiently small tip-sample distances, image forces induced by the strong tip field have been detected. MFM is used to investigate two Co-based longitudinal thin films. The roughness of the transition between head on oriented domains is compared.

• Luminescence in scanning tunneling microscopy on III–V nanostructures
S. F. Alvarado, Ph. Renaud, D. L. Abraham, C. Schönenberger, D. J. Arent, and H. P. Meier.
J. Vac. Sci. Technol. B, 9:409, 1991.

• Probing single charges by scanning force microscopy
C. Schönenberger and S. F. Alvarado.
Modern Phys. Lett. B, 5:871, 1991.

### 1990

• Observation of single charge carriers by force microscopy
C. Schönenberger and S. F. Alvarado.
Phys. Rev. Lett., 65:3162-3164, dec 1990.
[Abstract]

The scanning force microscope is used to deposit charge carriers on insulating Si3N4 films and to monitor their recombination. The charge decay shows up as a discontinuous staircase, demonstrating single carrier resolution. The decay is found to be controlled by thermionic emission.

• Understanding magnetic force microscopy
C. Schönenberger and S. F. Alvarado.
Z. Phys. B – Condensed Matter, 80:373-383, oct 1990.
[Abstract]

Magnetic force microscopy is a new method for imaging ferromagnetic domains with a high lateral resolution (10 nm). In this paper we give the basic tip parameters that have to be taken into account to achieve a quantitative image interpretation. For the electrochemically otched polycrystalline iron, nickel and cobalt wires, the tip-apex domain is found to be oriented along the tip axis, because of shape anisotropy. The stray field emerging from the tip apex is comparable to the size of the tip saturation field. The effective domain lengthL determines the image formation: the force due to magnetization patterns of scales which are large compared toL follow the point-dipole approximation. In the opposite case, a single-pole model is more appropriate. While a cobalt tip can be treated as an isolated domain, for nickel and iron a net polarization in the tip wire induced by the front apex-domain has to be considered. A new analytical theory provides an overall understanding of the image formation and allows the determination of the magnetic field vector and the estimation of its magnitude from measurements.

• Separation of magnetic and topographic effects in force microscopy
C. Schönenberger, S. F. Alvarado, S. E. Lambert, and I. L. Sanders.
J. Appl. Phys., 67:7278-7280, jun 1990.
[Abstract]

Several techniques are presented which allow magnetic force microscopy to be performed while simultaneously mapping the surface topographic features of a magnetic sample. The separation of magnetic and topographic features measured simultaneously with a scanning force microscope is made possible by an instrument based on a differential interferometer that can detect cantilever deflections of 0.005 nm at a frequency as low as 1 Hz. Two different applications are presented.

• Nanometer resolution in luminescence microscopy of III-V heterostructures
D. L. Abraham, A. Veider, C. Schönenberger, H. P. Meier, D. J. Arent, and S. F. Alvarado.
Appl. Phys. Lett., 56:1564-1566, feb 1990.
[Abstract]

In a scanning tunneling microscope experiment, the luminescence induced by the recombination of holes with electrons tunneling into cleaved (110) GaAs/AlGaAs heterostructures is used to image the interface region with nanometer resolution.

• STM and luminescence
D. L. Abraham, A. Veider, C. Schönenberger, D. J. Arent, H. P. Meier, and S. F. Alvarado.
Helv. Phys. Acta, 63:783, 1990.

### 1989

• Scanning tunneling microscopy as a tool to study surface roughness of sputtered thin films
C. Schönenberger, S. F. Alvarado, and C. Ortiz.
J. Appl. Phys., 66:4258-4261, jun 1989.
[Abstract]

A three‐dimensional image of the surface roughness of four conducting iron‐oxide Fe3O4 thin films was obtained using a scanning tunneling microscope. We obtain grain size and surface roughness of films deposited on Si(100) by reactive sputtering at different substrate temperatures. The apparent grain size lies between 10 and 50 nm, and depends on the substrate temperature and film thickness. We have also determined the scanning tunneling microscopy parameters (tip size and shape) to obtain ‘‘real’’ images (i.e., images without artifacts) of the films.

• A differential interferometer for force microscopy
C. Schönenberger and S. F. Alvarado.
Rev. Sci. Instr., 60:3131-3134, jun 1989.
[Abstract]

We present a polarizing optical interferometer especially developed for force microscopy. The deflections of the force-sensing cantilever are measured by means of the phase shift of two orthogonally polarized light beams, both reflected off the cantilever. This arrangement minimizes perturbations arising from fluctuations of the optical path length. Since the measured quantity is normalized versus the reflected intensity, the system is less sensitive to intensity fluctuations of the light source. The device is especially well suited to static force measurements. The total rms noise measured is < 0.01 Å in a frequency range from 1 Hz to 20 kHz.

### 1987

• Hall-effect and resistivity study of the heavy-fermion system URu$_{2}$Si$_{2}$
J. Schoenes, C. Schönenberger, J. J. M. Franse, and A. A. Menovsky.
Phys. Rev. B, 35(10):5375-5378, 1987.
[Abstract]

Hall-effect measurements from 2-300 K and resistivity measurements up to 1200 K are presented for different orientations of URu$_{2}$Si$_{2}$ single crystals. For H{\par}c a decomposition into an ordinary and an extraordinary Hall effect allows an estimate of the temperature dependence of the carrier concentration and provides evidence for the existence of a reconstruction of the Fermi surface at 17 K and the onset of coherence near 70 K. A third characteristic temperature, the single-impurity Kondo temperature T$_{K}$\~{}=370 K, is derived from the resistivity data at high temperatures.

• Transport Properties of the Heavy Fermion System URu2Si2
C. Schönenberger, C. Schoenes, and J. J. M. Franse.
Helvetica Physica Acta, 60(5-6):785-788, 1987.