We were one of the first group (if not the first) looking into QDs that are coupled to superconductors (SCs).1  Our devices were realized with carbon nanotubes CNTs and semiconducting nanowires (SNWs). In our early studies we were intrigued by the interplay between Kondo physics and superconducting proximity effect. We could demonstrate that if the Kondo temperature is larger than the superconducting pairing energy, the Kondo resonance would survive lading to an enhanced electrical conductance, signaling the transition to the Josephson effect.1,2 In the opposite limit, the gap of the SC would greatly suppress Kondo screening. These early studies have received great attention in recent years, since it is now possible to study hybrid QD devices with different coupling strength and also beyond two-terminals. We have recently demonstrated coherent two particle resonant and inelastic Andreev tunneling in a QD coupled to a superconducting and normal metal contact.3 We have also studied the temperature dependence and role of quasiparticles.4 Furthermore, we have found new intriguing correlations at higher coupling strengths to the SC in three terminal devices when Andreev-bound states are formed.

Bias-dependent differential conductance of a carbon-nanotube quantum dot connected to superconducting source and drain contacts. The boundary for quasiparticle transport at +/- 4 Δ is indicated by the white lines. Intriguing dispersion of Andreev-bound states, Kondo resonances (a particular strong one at Vgate~1V, and possibly also signs of multiple Andreev reflection (measurement by Alexander Eichler).

 

  1. M. R. Buitelaar, T. Nussbaumer and CS, Phys. Rev. Lett. 89 (25), 256801 (2002).
  2. A. Eichler, M. Weiss, S. Oberholzer, CS, A. L. Yeyati, J. C. Cuevas and A. Martin-Rodero, Phys. Rev. Lett. 99 (12), 126602 (2007).
  3. J. Gramich, A. Baumgartner and CS, Phys. Rev. Lett. 115 (21), 216801 (2015).
  4. J . Gramich, A. Baumgartner and CS, Appl. Phys. Lett. 108 (17), 172604 (2016).

Relevant papers (keyword: Supra):

2019

  • Intrinsically-limited timing jitter in molybdenum silicide superconducting nanowire single-photon detectors
    M. Caloz, B. Korzh, E. Ramirez, C. Schönenberger, R. J. Warburton, H. Zbinden, M. D. Shaw, and F. Bussières.
    J. Appl. Phys, 126:164501, oct 2019. [DOI] arXiv:1906.02073
    [Abstract]

    Recent progress in the development of superconducting nanowire single-photon detectors (SNSPDs) has delivered excellent performances, and has had a great impact on a range of research fields. The timing jitter, which denotes the temporal resolution of the detection, is a crucial parameter for many applications. Despite extensive work since their apparition, the lowest jitter achievable with SNSPDs is still not clear, and the origin of the intrinsic limits is not fully understood. Understanding its intrinsic behaviour and limits is a mandatory step toward improvements. Here, we report our experimental study on the intrinsically-limited timing jitter in molybdenum silicide (MoSi) SNSPDs. We show that to reach intrinsic jitter, several detector properties such as the latching current and the kinetic inductance of the devices have to be understood. The dependence on the nanowire cross-section and the energy dependence of the intrinsic jitter are exhibited as well as their fundamental limitations. System timing jitter of 6.0 ps at 532 nm and 10.6 ps at 1550 nm photon wavelength have been obtained.

  • Large spatial extension of the zero-energy Yu-Shiba-Rusinov state in magnetic field
    Z. Scherübl, G. Fülöp, C. P. Moca, J. Gramich, A. Baumgartner, P. Makk, T. Elalaily, C. Schönenberger, J. Nygard, G. Zaránd, and S. Csonka.
    submitted, aug 2019. arXiv:1906.08531
    [Abstract]

    Various promising qubit concepts have been put forward recently based on engineered superconductor (SC) subgap states like Andreev bound states, Majorana zero modes or the Yu-Shiba-Rusinov (Shiba) states. The coupling of these subgap states via a SC strongly depends on their spatial extension and is an essential next step for future quantum technologies. Here we investigate the spatial extension of a Shiba state in a semiconductor quantum dot coupled to a SC for the first time. With detailed transport measurements and numerical renormalization group calculations we find a remarkable more than 50 nm extension of the zero energy Shiba state, much larger than the one observed in very recent scanning tunneling microscopy (STM) measurements. Moreover, we demonstrate that its spatial extension increases substantially in magnetic field.

  • Spectroscopy of the superconducting proximity effect in nanowires using integrated quantum dots
    C. Jünger, A. Baumgartner, R. Delagrange, D. Chevallier, S. Lehmann, M. Nilsson, K. A. Dick, C. Thelander, and C. Schönenberger.
    Communications Physics, 2:76, july 2019. [DOI] arXiv:1812.06850
    [Abstract]

    The superconducting proximity effect has been the focus of significant research efforts over many YEARs and has recently attracted renewed interest as the basis of topologically non-trivial states in materials with a large spin orbit interaction, with protected boundary states useful for quantum information technologies. However, spectroscopy of these states is challenging because of the limited spatial and energetic control of conventional tunnel barriers. Here, we report electronic spectroscopy measurements of the proximity gap in a semiconducting indium arsenide (InAs) nanowire (NW) segment coupled to a superconductor (SC), using a spatially separated quantum dot (QD) formed deterministically during the crystal growth. We extract the characteristic parameters describing the proximity gap which is suppressed for lower electron densities and fully developed for larger ones. This gate-tunable transition of the proximity effect can be understood as a transition from the long to the short junction regime of subgap bound states in the NW segment. Our device architecture opens up the way to systematic, unambiguous spectroscopy studies of subgap bound states, such as Majorana bound states.

  • 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, feb 2019. [DOI] 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.

  • 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, feb 2019. [DOI] 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, oct 2018. [DOI] 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, sep 2018. [DOI] 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.

  • 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. [DOI] 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

  • 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. [DOI] 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.

  • 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. [DOI] 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.

2017

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

  • 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. [DOI] 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.

2016

  • A success story
    Christel Möller and Christian Schönenberger.
    Nature Nanotechnology, 11:908, Oct. 2016. [DOI] 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. [DOI] 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, March 2016. [DOI] arXiv:…
    [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.

2015

  • 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. [DOI] 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.