Ballistic graphene offers a promising platform for electron optical devices. We have developed a versatile technology that allows to suspend graphene and complement it with arbitrary bottom and top-gate structures.1 Using current annealing we demonstrated exceptional high nobilities approaching 102 m2/Vs. These suspended devices are ballistic over micrometer length scales and display intriguing interference patterns in the electrical conductance when different gate potentials and magnetic fields are applied.2 There are great similarities between the propagation of light in a dielectric and electrons in graphene, but also differences. In particular, a negative refractive index is straightforward to realize in graphene, but hard in optics. We have used pn junctions to define an electron waveguide by electrostatics3 and guide electrons in snake-states due to alternating cyclotron motion in a small magnetic field,4 to realized mirrors and momentum filters,2 beam splitters5 and Fabry-Perot-like cavities as well as more complex interferometers.

Bilayer graphene is an exciting material, widely extending the range of phenomena compared to monolayer graphene, due its massive nature and much larger interaction parameter. In bilayer graphene a gap can be opened by applying a potential difference between the two layers, for example. Furthermore, the eight-fold ground-state degeneracy of the zero-energy Landau level provides a large Hilbert space, where interaction is expected to lift the degeneracy resulting in novel composite particles. We have discovered that the ground-state in undoped ultraclean high-mobility bilayer graphene is gapped in the absence of magnetic and electric field.6 The new phase, which spontaneously appears driven by interactions, was latter assigned to an antiferromagnetic one. We have also seen recently that a gap appears, albeit much weaker, for graphene with four layers, but it is absent in graphene with a odd layer number.

In current projects we use high-quality h-BN encapsulated graphene and study superlattice effects induced by lattice of the substrate.7 We further work extensively on superconducting graphene devices with side contacts. We have realized Josephson junctions with sputter superconducting films. We also explore isospin currents due to valley effects using pn junctions and suspended graphene where we aim to control the strain in latter with the goal to realize a valley physics with a pseudomagnetic field.

Funding: NCCR-QSIT, SNI

1. R. Maurand, P. Rickhaus, P. Makk, S. Hess, E. Tovari, C. Handschin, M. Weiss and CS, Carbon 79, 486 (2014).
2. P. Rickhaus, R. Maurand, M. H. Liu, M. Weiss, K. Richter and CS, Nat. Commun. 4, 2342 (2013).
3. P. Rickhaus, M. H. Liu, P. Makk, R. Maurand, S. Hess, S. Zihlmann, M. Weiss, K. Richter and CS, Nano Lett. 15 (9), 5819-5825 (2015).
4. P. Rickhaus, P. Makk, M. H. Liu, E. Tovari, M. Weiss, R. Maurand, K. Richter and CS, Nat. Commun. 6, 6470 (2015).
5. P. Rickhaus, P. Makk, M. H. Liu, K. Richter and CS, Appl. Phys. Lett. 107 (25), 251901 (2015).
6. F. Freitag, J. Trbovic, M. Weiss and CS, Phys. Rev. Lett. 108 (6), 076602 (2012).
7. C. Handschin, P.Makk, P. Rickhaus, M.-H. Liu, K. Watanabe, T. Taniguchi, K. Richter, C. Schönenberger, Nano Lett. 17, 328 (2016).

Relevant papers (keyword: GRAPHENE):

##### 2022
• Phase-dependent microwave response of a graphene Josephson junction
R. Haller, G. Fülöp, D. Indolese, J. Ridderbos, R. Kraft, Luk Yi Cheung, J. H. Ungerer, K. Watanabe, T. Taniguchi, D. Beckmann, R. Danneau, P. Virtanen, and C. Schönenberger.
Phys. Rev. Research  4, 13198 (2022)
[arXiv:2108.00989 ] [ Open Data ] [Abstract ]

Gate-tunable Josephson junctions embedded in a microwave environment provide a promising platform to in-situ engineer and optimize novel superconducting quantum circuits. The key quantity for the circuit design is the phase-dependent complex admittance of the junction, which can be probed by sensing an rf SQUID with a tank circuit. Here, we investigate a graphene-based Josephson junction as a prototype gate-tunable element enclosed in a SQUID loop that is inductively coupled to a superconducting resonator operating at 3 GHz. With a concise circuit model that describes the dispersive and dissipative response of the coupled system, we extract the phase-dependent junction admittance corrected for self-screening of the SQUID loop. We decompose the admittance into the current-phase relation and the phase-dependent loss and as these quantities are dictated by the spectrum and population dynamics of the supercurrent-carrying Andreev bound states, we gain insight to the underlying microscopic transport mechanisms in the junction. We theoretically reproduce the experimental results by considering a short, diffusive junction model that takes into account the interaction between the Andreev spectrum and the electromagnetic environment, from which we deduce a lifetime of ~17 ps for non-equilibrium populations.

• 2D materials shrink superconducting qubits
C. Schönenberger.
Nature Materials (News & Views)  21, 381 (2022)
[Abstract ]

The exceptional quality of hexagonal boron nitride crystals that can be cleaved into few layers provides ultrathin dielectrics, thereby opening a route to ultrasmall capacitors with large capacitances. With such capacitors, the superconducting transmon qubit is scaled down by orders of magnitude. % Associated Content: `Hexagonal boron nitride as a low-loss dielectric for superconducting quantum circuits and qubits’ by J. J.-I. Wang et al. Nature Materials 21, 398–403 (2022)

##### 2021
• Boosting proximity spin orbit coupling in graphene/WSe2 heterostructures via hydrostatic pressure
B. Fülöp, A. Márffy, S. Zihlmann, M. Gmitra, E. Tóvári, B. Szentpéteri, M. Kedves, K. Watanabe, T. Taniguchi, J. Fabian, C. Schönenberger, P. Makk, and S. Csonka.
npj 2D Materials and Applications  5, 82 (2021)
[arXiv:2103.13325 ] [ Open Data ] [Abstract ]

Van der Waals heterostructures composed of multiple few layer crystals allow the engineering of novel materials with predefined properties. As an example, coupling graphene weakly to materials with large spin orbit coupling (SOC) allows to engineer a sizeable SOC in graphene via proximity effects. The strength of the proximity effect depends on the overlap of the atomic orbitals, therefore, changing the interlayer distance via hydrostatic pressure can be utilized to enhance the interlayer coupling between the layers. In this work, we report measurements on a graphene/WSe2 heterostructure exposed to increasing hydrostatic pressure. A clear transition from weak localization to weak anti-localization is visible as the pressure increases, demonstrating the increase of induced SOC in graphene.

• New method of transport measurements on van der Waals heterostructures under pressure
B. Fülöp, A. Márffy, E. Tóvári, M. Kedves, S. Zihlmann, D. Indolese, Z. Kovács-Krausz, K. Watanabe, T. Taniguchi, C. Schönenberger, Kézsmárki I. P. Makk, and S. Csonka.
J. Apl. Phys.  130, 64303 (2021)
[arXiv:2103.14617 ] [ Open Data ] [Abstract ]

The interlayer coupling, which has a strong influence on the properties of van der Waals heterostructures, strongly depends on the interlayer distance. Although considerable theoretical interest has been demonstrated, experiments exploiting a variable interlayer coupling on nanocircuits are scarce due to the experimental difficulties. Here, we demonstrate a novel method to tune the interlayer coupling using hydrostatic pressure by incorporating van der Waals heterostructure based nanocircuits in piston-cylinder hydrostatic pressure cells with a dedicated sample holder design. This technique opens the way to conduct transport measurements on nanodevices under pressure using up to 12 contacts without constraints on the sample at the fabrication level. Using transport measurements, we demonstrate that a hexagonal boron nitride capping layer provides a good protection of van der Waals heterostructures from the influence of the pressure medium, and we show experimental evidence of the influence of pressure on the interlayer coupling using weak localization measurements on a transitional metal dichalcogenide/graphene heterostructure.

• Superconducting contacts to a monolayer semiconductor
M. Ramezani, Correa I. Sampaio, K. Watanabe, T. Taniguchi, C. Schönenberger, and A. Baumgartner.
Nano Letters  21, 5614 (2021)
[arXiv:2102.06227 ] [ Open Data ] [Abstract ]

We demonstrate superconducting vertical interconnect access (VIA) contacts to a mono-layer of molybdenum disulfide (MoS2), a layered semiconductor with highly relevant elec-tronic and optical properties. As a contact material we use MoRe, a superconductor with a high critical magnetic field and high critical temperature. The electron transport is mostly dominated by a single superconductor/normal conductor junction with a clear superconductor gap. In addition, we find MoS2 regions that are strongly coupled to the superconductor, resulting in resonant Andreev tunneling and junction dependent gap characteristics, suggesting a superconducting proximity effect. Magnetoresistance measurements show that the band-structure and the high intrinsic carrier mobility remain intact in the bulk of the MoS2. This type of VIA contact is applicable to a large variety of layered materials and superconducting

• Global strain-induced scalar potential in graphene devices
L. Wang, A. Baumgartner, P. Makk, S. Zihlmann, B. S. Varghese, D. I. Indolese, K. Watanabe, T. Taniguchi, and C. Schönenberger.
Comm. Phys.  4, 147 (2021)
[arXiv:2009.03035 ] [ Open Data ] [Abstract ]

By mechanically distorting a crystal lattice it is possible to engineer the electronic and optical properties of a material. In graphene, one of the major effects of such a distortion is an energy shift of the Dirac point, often described as a scalar potential. We demonstrate how such a scalar potential can be generated systematically over an entire electronic device and how the resulting changes in the graphene work function can be detected in transport experiments. Combined with Raman spectroscopy, we obtain a characteristic scalar potential consistent with recent theoretical estimates. This direct evidence for a scalar potential on a macroscopic scale due to deterministically generated strain in graphene paves the way for engineering the optical and electronic properties of graphene and similar materials by using external strain.

##### 2020
• Out-of-plane corrugations in graphene based van der Waals heterostructures
S. Zihlmann, P. Makk, M. K. Rehmann, L. Wang, M. Kedves, D.Indolese, K. Watanabe, T. Taniguchi, D. M. Zumbühl, and C. Schönenberger.
Phys. Rev. B  102, 195404 (2020)
[arXiv:2004.02690 ] [ Open Data ] [Abstract ]

Two dimensional materials are usually envisioned as flat, truly 2D layers. However out-of-plane corrugations are inevitably present in these materials. In this manuscript, we show that graphene flakes encapsulated between insulating crystals (hBN, WSe2), although having large mobilities, surprisingly contain out-of-plane corrugations. The height fluctuations of these corrugations are revealed using weak localization measurements in the presence of a static in-plane magnetic field. Due to the random out-of-plane corrugations, the in-plane magnetic field results in a random out-of-plane component to the local graphene plane, which leads to a substantial decrease of the phase coherence time. Atomic force microscope measurements also confirm a long range height modulation present in these crystals. Our results suggest that phase coherent transport experiments relying on purely in-plane magnetic fields in van der Waals heterostructures have to be taken with serious care.

• Compact SQUID realized in a double layer graphene heterostructure
D. I. Indolese, P. Karnatak, A. Kononov, R. Delagrange, R. Haller, L. Wang, P. Makk, K. Watanabe, T. Taniguchi, and C. Schönenberger.
Nano Letters  20, 7129–7135 (2020)
[arXiv:2006.05522 ] [ Open Data ] [Abstract ]

Two-dimensional systems that host one-dimensional helical states are exciting from the perspective of scalable topological quantum computation when coupled with a superconductor. Graphene is particularly promising for its high electronic quality, versatility in van der Waals heterostructures and its electron and hole-like degenerate 0$th$ Landau level. Here, we study a compact double layer graphene SQUID (superconducting quantum interference device), where the superconducting loop is reduced to the superconducting contacts, connecting two parallel graphene Josephson junctions. Despite the small size of the SQUID, it is fully tunable by independent gate control of the Fermi energies in both layers. Furthermore, both Josephson junctions show a skewed current phase relationship, indicating the presence of superconducting modes with high transparency. In the quantum Hall regime we measure a well defined conductance plateau of 2$e^2/h$ an indicative of counter propagating edge channels in the two layers. Our work opens a way for engineering topological superconductivity by coupling helical edge states, from graphene’s electron-hole degenerate 0$th$ Landau level via superconducting contacts.

• Experimental demonstration of the suppression of optical phonon splitting in 2D materials by Raman spectroscopy
De M. Luca, X. Cartoixa, D. Indolese, J. Martín-Sánchez, K. Watanabe, T. Taniguchi, C. Schönenberger, R. Trotta, R. Rurali, and I. Zardo.
2D Materials  7 (2020)
[arXiv:2009.07618 ] [Abstract ]

Raman spectroscopy is one of the most extended experimental techniques to investigate thin-layered 2D materials. For a complete understanding and modeling of the Raman spectrum of a novel 2D material, it is often necessary to combine the experimental investigation to density-functional-theory calculations. We provide the experimental proof of the fundamentally different behavior of polar 2D vs 3D systems regarding the effect of the dipole−dipole interactions, which in 2D systems ultimately lead to the absence of optical phonons splitting, otherwise present in 3D materials. We demonstrate that non-analytical corrections (NACs) should not be applied to properly model the Raman spectra of few-layered 2D materials, such as WSe2 and h-BN, corroborating recent theoretical predictions [Nano Lett. 2017, 17 (6), 3758-3763]. Our findings are supported by measurements performed on tilted samples that allow increasing the component of photon momenta in the plane of the flake, thus unambiguously setting the direction of an eventual NAC. We also investigate the influence of the parity of the number of layers and of the type of layer-by-layer stacking on theeffect of NACs on the Raman spectra.

• Mobility enhancement in graphene by in situ reduction of random strain fluctuations
L. Wang, P. Makk, S. Zihlmann, A. Baumgartner, D. I. Indolese, K. Watanabe, T. Taniguchi, and C. Schönenberger.
Phys. Rev. Lett.  124, 157701 (2020)
[arXiv:1909.13484 ] [ Open Data ] [Abstract ]

Microscopic corrugations are ubiquitous in graphene even when placed on atomically flat substrates. These result in random local strain fluctuations limiting the carrier mobility of high quality hBN-supported graphene devices. We present transport measurements in hBN-encapsulated devices where such strain fluctuations can be in situ reduced by increasing the average uniaxial strain. When ∼0.2\% of uniaxial strain is applied to the graphene, an enhancement of the carrier mobility by ∼35\% is observed while the residual doping reduces by ∼39\%. We demonstrate a strong correlation between the mobility and the residual doping, from which we conclude that random local strain fluctuations are the dominant source of disorder limiting the mobility in these devices. Our findings are also supported by Raman spectroscopy measurements.

##### 2019
• In-situ strain tuning in hBN-encapsulated graphene electronic devices
L. Wang, S. Zihlmann, A. Baumgartner, J. Overbeck, K. Watanabe, T. Taniguchi, P. Makk, and C. Schönenberger.
Nano Letters  19, 4097-4102 (2019)
[arXiv:1904.06737 ] [ Open Data ] [Abstract ]

Using a simple setup to bend a flexible substrate, we demonstrate deterministic and reproducible in-situ strain tuning of graphene electronic devices. Central to this method is the full hBN encapsulation of graphene, which preserves the exceptional quality of pristine graphene for transport experiments. In addition, the on-substrate approach allows one to exploit strain effects in the full range of possible sample geometries and at the same time guarantees that changes in the gate capacitance remain negligible during the deformation process. We use Raman spectroscopy to spatially map the strain magnitude in devices with two different geometries and demonstrate the possibility to engineer a strain gradient, which is relevant for accessing the valley degree of freedom with pseudo-magnetic fields. Comparing the transport characteristics of a suspended device with those of an on-substrate device, we demonstrate that our new approach does not suffer from the ambiguities encountered in suspended devices

• GHz nanomechanical resonator in an ultraclean suspended graphene p-n junction
Minkyung Jung, P. Rickhaus, S. Zihlmann, A. Eichler, P. Makk, and C. Schönenberger.
Nanoscale  11, 4355 (2019)
[arXiv:1812.06412 ] [Abstract ]

We demonstrate high-frequency mechanical resonators in ballistic graphene p–n junctions. Fully suspended graphene devices with two bottom gates exhibit ballistic bipolar behavior after current annealing. We determine the graphene mass density and built-in tension for different current annealing steps by comparing the measured mechanical resonant response to a simplified membrane model. We consistently find that after the last annealing step the mass density compares well with the expected density of pure graphene. In a graphene membrane with high built-in tension, but still of macroscopic size with dimensions 3 × 1 micrometer^2, a record resonance frequency of 1.17 GHz is observed after the final current annealing step. We further compare the resonance response measured in the unipolar with the one in the bipolar regime. Remarkably, the resonant signals are strongly enhanced in the bipolar regime. This enhancement is caused in part by the Fabry-Pérot resonances that appear in the bipolar regime and possibly also by the photothermoelectric effect that can be very pronounced in graphene p–n junctions under microwave irradiation.

• New generation of Moiré superlattices in doubly aligned hBN/graphene/hBN heterostructures
L. Wang, S. Zihlmann, Ming-Hao Liu, P. Makk, K. Watanabe, T. Taniguchi, A. Baumgartner, and C. Schönenberger.
Nano Letters  19, 2371-2376 (2019)
[arXiv:1812.10031 ] [Abstract ]

The specific rotational alignment of two-dimensional lattices results in a moiré superlattice with a larger period than the original lattices and allows one to engineer the electronic band structure of such materials. So far, transport signatures of such superlattices have been reported for graphene/hBN and graphene/graphene systems. Here we report moiré superlattices in fully hBN encapsulated graphene with both the top and the bottom hBN aligned to the graphene. In the graphene, two different moiré superlattices form with the top and the bottom hBN, respectively. The overlay of the two superlattices can result in a third superlattice with a period larger than the maximum period (\SI{14}{nm}) in the graphene/hBN system, which we explain in a simple model. This new type of band structure engineering allows one to artificially create an even wider spectrum of electronic properties in two-dimensional materials.

• Non-equilibrium properties of graphene probed by superconducting tunnel spectroscopy
S. Zihlmann, P. Makk, S. Castillas, J. Gramich, K. Thodkar, S. Caneva, R. Wang, S. Hofmann, and C. Schönenberger.
Phys. Rev. B  99, 75419 (2019)
[arXiv:1811.08746 ] [Abstract ]

We report on non-equilibrium properties of graphene probed by superconducting tunnel spectroscopy. A hexagonal boron nitride (hBN) tunnel barrier in combination with a superconducting Pb contact is used to extract the local energy distribution function of the quasiparticles in graphene samples in different transport regimes. In the cases where the energy distribution function resembles a Fermi-Dirac distribution, the local electron temperature can directly be accessed. This allows us to study the cooling mechanisms of hot electrons in graphene. In the case of long samples (device length L much larger than the electron-phonon scattering length le−ph), cooling through acoustic phonons is dominant. We find a cross-over from the dirty limit with a power law T3 at low temperature to the clean limit at higher temperatures with a power law T4 and a deformation potential of 13..3 eV. For shorter samples, where L is smaller than le−ph but larger than the electron-electron scattering length le−e, the well-known cooling through electron out-diffusion is found. Interestingly, we find strong indications of an enhanced Lorenz number in graphene. We also find evidence of a non-Fermi-Dirac distribution function, which is a result of non-interacting quasiparticles in very short samples

##### 2018
• 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.
Phys. Rev. Appl.  10, 44053 (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.

• 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.
Phys. Rev. Lett.  121, 137701 (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.

• Observation of High Accuracy Resitance Quantization in CVD Graphene
K. Thodkar, C. Schönenberger, M. Calame, F. Lüönd, and Overrney F. B. ad Jeanneret.
2018 Conference on Precision Electromagnetic Measurements (CPEM 2018)  pages 1-2 (2018)
[Abstract ]

A prime technique to produce graphene is Chemical Vapor Deposition (CVD). In this paper, the first observation of high accuracy resistance quantization in CVD graphene samples grown on polycrystalline copper foils is shown. High precision measurements performed using a cryogenic current comparator reveal a resistance quantization accuracy of 100 parts in 10^9.

• 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  97, 75434 (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 (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.

• 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 (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 (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
• 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 (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 (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. Appl.  7(5), 54015 (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.

• 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 (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.

##### 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 (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, and C. Schönenberger.
Nano Letters  16(11), 6988 (2016)
[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.

• 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 (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.

• 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 (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.

• 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 (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.

• 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, 11480 (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.

##### 2015
• Gate tuneable beamsplitter in ballistic graphene
P. Rickhaus, P. Makk, M. -H. Liu, K. Richter, and C. Schönenberger.
Applied Physics Letters  107, 251901 (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

• Gate tuneable beamsplitter in ballistic graphene
P. Makk.
Nature Physics  11, 894-895 (2015)

• 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 (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.

• 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, 36601 (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.

• 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 (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.

• 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(5819) (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.

• 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, 30202 (2015)

##### 2014
• CVD Graphene 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 (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.

• 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 (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.

• 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 (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.

• High-yield fabrication of nm-sized gaps in monolayer CVD graphene
C. Nef, L. Pósa, P. Makk, W. Fu, A. Halbritter, C. Schönenberger, and M. Calame.
Nanoscale  6, 7249-7254 (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.

• 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 (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.

• 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 (2014)
[arXiv:1403.6976 ] [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.

##### 2013
• 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, 71601 (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.

• 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 (2013)
[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.

• 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, 3193-3198 (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

• Spin Symmetry of the Bilayer Graphene Ground State
F. Freitag, M. Weiss, R. Maurand, J. Trbovic, and C. Schönenberger.
Phys. Rev. B  87, 161402 (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

##### 2012
• Homogeneity of Bilayer Graphene
F. Freitag, M. Weiss, R. Maurand, J. Trbovic, and C. Schönenberger.
Solid State Communications  152, 2053-2057 (2012)
[arXiv:1207.4424 ] [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.

• Spontaneously Gapped Ground State in Suspended Bilayer Graphene
F. Freitag, J. Trbovic, M. Weiss, and C. Schönenberger.
Phys. Rev. Lett.  108, 76602 (2012)

• Quantum Hall Effect in Graphene with Superconducting Electrodes
P. Rickhaus, M. Weiss, L. Marot, and C. Schönenberger.
Nano Letters  12, 1942 (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, 2649 (2011)

##### 2010
• Superconductivity-enhanced conductance fluctuations in few layer graphene
J. Trbovic, N. Minder, F. Freitag, and C. Schönenberger.
Nanotechnology  21, 274005 (2010)