suspended CVD graphene
Carbon nanotubes (CNTs) are almost ideal one-dimensional conductors. However, potential variation caused likely by charge tarps in the substrate induced backscatering on a length scale of a few 100 nm. There are two strategies to overcome this problem: in one the CNT is suspended and in the other it is placed on another substrate. Of particular interest is h-BN, since this substrate yielded much better charge transport mobilities in graphene when place on h-BN instead on an oxidized Si wafer. We have developed a deterministic approach to place single CNTs suspended over contacts and gate arrays and tested CNT on h-BN. In order to go beyond the state-of-the-art we have started to fabricate three-terminal CNT devices with weak contacts, i.e. tunneling contacts using monolayers of h-BN as barriers. Three weakly coupled contacts on a single section of CNT, a quantum dots, allows to obtain all coupling parameters unambiguously. We are also interested to use grow CNT made from the isotope 13C providing a nuclear spin. Due to strong electron-electron interaction the indirect spin-spin coupling can be strongly enhanced leading to a phase transition to a nuclear spin helix at low temperatures. Consequently, the spin in the conduction band is also polarized providing a „natural“ system to test Majorana fermion physics by coupling the CNT to a superconductor.
Current challenges are: weakly coupled CNTs to allow for high resolution tunneling spectroscopy, measure the spin-lifetime in 12C and 13C CNTs using spin blockade, coupling CNTs to superconductors and measure Andreev bound state in magnetic field.
Funding: ERC-QUEST, SNF
- 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
Quantum dots (QDs) investigated through electron transport measurements often exhibit varying, state-dependent tunnel couplings to the leads. Under specic 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 dierential 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.
- 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
Andreev bound states (ABSs) are well-dened 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
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.
- A success story
Christel Möller and Christian Schönenberger.
Nature Nanotechnology, 11:908, Oct. 2016. arXiv:…
- Subgap resonant quasiparticle transport in normal-superconductor quantum dot devices
J. Gramich, A. Baumgartner, and C. Schönenberger.
Appl. Phys. Lett., 108(17):172604, April 2016. arXiv:1601.00672
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, March 2016. arXiv:…
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.
- Full characterization of a carbon nanotube parallel double quantum dot
G. Abulizi, A. Baumgartner, and C. Schönenberger.
Physica Status Solidi B, 253(12):2428, 2016. arXiv:1605.02300v1
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.
- 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, November 2015. arXiv:1504.05693
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.
- 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
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
- 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, 115:227003, 2015. arXiv:1507.01036
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.
- Resonant and Inelastic Andreev Tunneling Observed on a Carbon Nanotube Quantum Dot
J. Gramich, A. Baumgartner, and C. Schönenberger.
Physical Review Letters, 115:216801, 2015. arXiv:1507.00526
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.
- 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, 2015. arXiv:1505.04681
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.
- 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, June 2014. arXiv:1406.0897
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.
- 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, 2014.
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.
- Nonlocal spectroscopy of Andreev bound states
J. Schindele, A. Baumgartner, R. Maurand, M. Weiss, and C. Schönenberger.
Phys. Rev. B, 89:45422, 2014.
- 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.
- Near-Unity Cooper Pair Splitting Efficiency
J. Schindele, A. Baumgartner, and C. Schönenberger.
Phys. Rev. Lett., 109:157002, 2012.
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.
- Gate-tunable split Kondo effect in a carbon nanotube quantum dot
A. Eichler, M. Weiss, and C. Schönenberger.
Nanotechnology, 22:265201, 2011.
- Permalloy-based carbon nanotube spin-valve
H. Aurich, A. Baumgartner, F. Freitag, A. Eichler, J. Trbovic, and C. Schönenberger.
App. Phys. Lett, 97:153116, September 2010.
- 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), 2009.
- 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, 2008.
- 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, 2007.
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.
- 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, 2007.
- 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, April 2006.
- 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, 2006.
- 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, 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, 2006.
- Charge and Spin Transport in Carbon Nanotubes
Semicond. Sci. Technol., 21:S1-S9, 2006.
- 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.
- 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, March 2005.
- On the Kondo effect in carbon nanotubes at half-filling
B. Babić, T. Kontos, and C. Schönenberger.
Phys. Rev. B, 70:235419, 2004.
- Observation of Fano-Resonances in Single-Wall Carbon Nanotubes
B. Babić and C. Schönenberger.
Phys. Rev. B, 70:195408, 2004.
- Conductance properties of nanotubes coupled to superconducting leads: signatures of Andreev states dynamics
E. Vecino, M. Buitelaar, A. Martín-Rodero, C. Schönenberger, and Levy A. Yeyati.
Solid-State Communications 131, 625 (2004), 131:625, cond-mat/0406240 2004.
- Quantum dot coupled to a normal and a superconducting lead
M. R. Gräber, T. Nussbaumer, W. Belzig, and C. Schönenberger.
Nanotechnology, 15:S479, 2004.
- 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, August 2003.
- 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, 2003.
- Ambipolar field-effect transistor on as-grown single-wall carbon nanotube
B. Babić, M. Iqbal, and C. Schönenberger.
Nanotechnology, 14:327-331, 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, 2003.
- 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, April 2002.
- A quantum dot in the Kondo regime coupled to superconductors
M. \. R. \. Buitelaar, T. \. Nussbaumer, and C. \. Schönenberger.
Phys. Rev. Lett., 89(25):256801, 2002.
- 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~15 2001.
- Carbon nanotubes, materials for the future
L. Forró and C.~Schönenberger.
Europhysics news, 32(3):86-90, May/June 2001.
- The Electrochemical Nanotube Field-Effect Transistor
M. Krüger, M. Buitelaar, T. Nussbaumer, C.~Schönenberger, and L. \. Forró.
App. Phys. Lett., 78(9):1291-1293, Feb.~2001 2001.
- Comment on `Magnetoresistance and differential conductance in multiwalled carbon nanotubes’
C. Schönenberger and A. Bachtold.
Phys. Rev. B, 64:157401, 2001.
- Physics of Multiwall-Carbon Nanotubes
C. Schönenberger and L. Forró.
Physics World, 13(6):37-41, June 2000.
- Electric Properties of Multiwall Carbon Nanotubes
Proceedings of the Int. School of Physics, Enrico Fermi, Nuovo Cimento, June 2000.
- Interference and interactions in multiwall nanotubes
C.~Strunk, A.~Bachtold, T.~Nussbaumer, and C.~Schönenberger.
Physica B, 280(1-4):384-385, 2000.
The electrical properties of single multiwall nanotubes (NTs) have been investigated in parallel and perpendicular magnetic field. Quantum interference phenomena like weak localization, Aharonov-Bohm effect, UCF, and nonlocal resistance contributions prove that NTs are mesoscopic (phase-coherent) objects at low temperature. The relatively large elastic-scattering length, inferred from our data, suggests that our NTs are 1d quasi-ballistic conductors where long-range coulomb interactions should be important. This is further substantiated by the temperature dependence of the resistance and by tunneling spectroscopy displaying a pronounced zero-bias anomaly.
- Interference and Interaction in Multiwall Carbon Nanotubes
C. Schönenberger, A. Bachtold, C. Strunk, J. -P. Salvetat, and L. Forró.
Appl. Phys. A, 69:283, 1999.
- 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, 1998.