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):

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

  • Subgap resonant quasiparticle transport in normal-superconductor quantum dot devices
    J. Gramich, A. Baumgartner, and C. Schönenberger.
    Applied Physics Letters, 108(17):172604, apr 2016. [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):62-63, mar 2016. [DOI]
    [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(21):216801, nov 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.

2014

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

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

2010

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

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

2004

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

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

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

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

2003

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

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

2002

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

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

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

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

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

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

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

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

2000

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

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

1999

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

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