Spectroscopy of the local density of states in nanowires using integrated quantum dots (original) (raw)
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Normal-state conductance used to probe superconducting tunnel junctions for quantum computing
Superconductor Science and Technology, 2010
Here we report normal-state conductance measurements of three different types of superconducting tunnel junctions that are being used or proposed for quantum computing applications: p-Al/a-AlO/p-Al, e-Re/e-AlO/p-Al, and e-V/e-MgO/p-V, where p stands for polycrystalline, e for epitaxial, and a for amorphous. All three junctions exhibited significant deviations from the parabolic behavior predicted by the WKB approximation models. In the p-Al/a-AlO/p-Al junction, we observed enhancement of tunneling conductances at voltages matching harmonics of Al-O stretching modes. On the other hand, such Al-O vibration modes were missing in the epitaxial e-Re/e-AlO/p-Al junction. This suggests that absence or existence of the Al-O stretching mode might be related to the crystallinity of the AlO tunnel barrier and the interface between the electrode and the barrier. In the e-V/e-MgO/p-V junction, which is one of the candidate systems for future superconducting qubits, we observed suppression of the density of states at zero bias. This implies that the interface is electronically disordered, presumably due to oxidation of the vanadium surface underneath the MgO barrier, even if the interface was structurally well ordered, suggesting that the e-V/e-MgO/p-V junction will not be suitable for qubit applications in its present form. This also demonstrates that the normal-state conductance measurement can be effectively used to screen out low quality samples in the search for better superconducting tunnel junctions.
Hard gap in epitaxial semiconductor-superconductor nanowires
Nature nanotechnology, 2015
Many present and future applications of superconductivity would benefit from electrostatic control of carrier density and tunnelling rates, the hallmark of semiconductor devices. One particularly exciting application is the realization of topological superconductivity as a basis for quantum information processing. Proposals in this direction based on the proximity effect in semiconductor nanowires are appealing because the key ingredients are currently in hand. However, previous instances of proximitized semiconductors show significant tunnelling conductance below the superconducting gap, suggesting a continuum of subgap states-a situation that nullifies topological protection. Here, we report a hard superconducting gap induced by the proximity effect in a semiconductor, using epitaxial InAs-Al semiconductor-superconductor nanowires. The hard gap, together with favourable material properties and gate-tunability, makes this new hybrid system attractive for a number of applications, a...
Hard Gap in Epitaxial Superconductor-Semiconductor Nanowires
arXiv (Cornell University), 2014
Many present and future applications of superconductivity would benefit from electrostatic control of carrier density and tunneling rates, the hallmark of semiconductor devices. One particularly exciting application is the realization of topological superconductivity as a basis for quantum information processing. Proposals in this direction based on proximity effect in semiconductor nanowires are appealing because the key ingredients are currently in hand. However, previous instances of proximitized semiconductors show significant tunneling conductance below the superconducting gap, suggesting a continuum of subgap states---a situation that nullifies topological protection. Here, we report a hard superconducting gap induced by proximity effect in a semiconductor, using epitaxial Al-InAs superconductor-semiconductor nanowires. The hard gap, along with favorable material properties and gate-tunability, makes this new hybrid system attractive for a number of applications, as well as fundamental studies of mesoscopic superconductivity.
2021
Deividas Sabonis, 2, ∗ David van Zanten, 2, ∗ Judith Suter, 2 Torsten Karzig, Dmitry I. Pikulin, 4 Jukka I. Väyrynen, 5 Eoin O’Farrell, 2 Davydas Razmadze, 2 Peter Krogstrup, 6 and Charles M. Marcus 2 Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark Microsoft Quantum Lab–Copenhagen, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark Microsoft Quantum, Station Q, University of California, Santa Barbara, California 93106-6105, USA Microsoft Quantum, Redmond, Washington 98052, USA Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana 47907, USA Microsoft Quantum Materials Lab–Copenhagen, 2800 Lyngby, Denmark
Statistics of Tunneling Events in Three-Terminal Hybrid Devices with Quantum Dot
Acta Physica Polonica A, 2018
We investigate statistics of the tunneling events in the short time limit in terms of the waiting time distribution (WTD), defined as the probability for a delay time between two subsequent transitions of particles, and consider it for a quantum dot (QD) strongly coupled to a superconducting and weakly coupled to two normal electrodes. Our study focuses on the WTD in the subgap transport, when coherent exchange of the Cooper pairs occurs between the QD and the superconductor. The dynamics can be described in terms of a Markovian generalized master equation for the reduced density matrix. We observe coherent oscillations between the Andreev bound states in the correlated jumps, both for the local and non-local WTDs. In addition the analysis of the transient currents give us some insight into dominant relaxation processes in short time scales.
Direct transport between superconducting subgap states in a double quantum dot
Physical review, 2022
We demonstrate direct transport between two opposing sets of Yu-Shiba-Rusinov (YSR) subgap states realized in a double quantum dot. This sub-gap transport relies on intrinsic quasiparticle relaxation, but the tunability of the device allows us to explore also an additional relaxation mechanism based on charge transferring Andreev reflections. The transition between these two relaxation regimes is identified in the experiment as a marked gate-induced stepwise change in conductance. We present a transport calculation, including YSR bound states and multiple Andreev reflections alongside with quasiparticle relaxation, due to a weak tunnel coupling to a nearby normal metal, and obtain excellent agreement with the data.
Physical Review B
The recent experimental observations of decaying energy oscillations with increasing magnetic field in semiconductor-superconductor Majorana nanowires is in contrast with the theoretical expectations based on the presence of Majorana zero modes localized at the ends of the system. These observations have been recently theoretically justified by considering a position-dependent spin-orbit coupling that can emerge due to a tunnel gate. Here, we show that the window in parameter space where this phenomenology occurs is vanishingly small when compared to the parameter region where Majorana oscillations should increase in amplitude with the applied magnetic field. Further, including a position-dependent effective potential that is also induced due to a tunnel gate practically removes this small window. Using extensive numerical calculations, we show that, as expected, increasing amplitude oscillations of the hybridization energy represent a generic property of topological Majorana zero modes, while decreasing amplitude oscillations are a generic property of low-energy trivial Andreev bound states, recently called partially separated Andreev bound states. By averaging over several realistic parameter configurations, we identify robust features of the hybridization energy that can be observed in a typical differential conductance experiment without fine-tuning the control parameters.
Physical Review X, 2020
Following significant progress in the visualization and characterization of Majorana end modes in hybrid systems of semiconducting nanowires and superconducting islands, much attention is devoted to the investigation of the electronic structure at the buried interface between the semiconductor and the superconductor. The properties of that interface and the structure of the electronic wave functions that occupy it determine the functionality and the topological nature of the superconducting state induced therein. Here we study this buried interface by performing spectroscopic mappings of superconducting aluminum islands epitaxially grown in situ on indium arsenide nanowires. We find unexpected robustness of the hybrid system as the direct contact with the aluminum islands does not lead to any change in the chemical potential of the nanowires, nor does it induce a significant band bending in their vicinity. We attribute this to the presence of surface states bound to the facets of the nanowire. Such surface states, which are present also in bare nanowires prior to aluminum deposition, pin the Fermi level, thus rendering the nanowires resilient to surface perturbations. The aluminum islands further display Coulomb blockade gaps and peaks that signify the formation of a resistive tunneling barrier at the InAs-Al interface. The extracted interface resistivity, ρ ≈ 1.3 × 10 −6 Ω cm 2 , will allow us to proximity induce superconductivity with negligible Coulomb blockade effects by islands with interface area as small as 0.01 μm 2. At low energies we identify a potential energy barrier that further suppresses the transmittance through the interface. A corresponding barrier exists in bare semiconductors between surface states and the accumulation layer, induced to maintain charge neutrality. Our observations elucidate the delicate interplay between the resistive nature of the InAs-Al interface and the ability to proximitize superconductivity and tune the chemical potential in semiconductor-superconductor hybrid nanowires.
Study of Pseudogap State in NbN using Scanning Tunneling Spectroscopy
AIP Conference …, 2011
We present scanning tunneling spectroscopy, transport and magneto-transport measurements on disordered epitaxial thin films of NbN as a function of disorder. The studies show a spatial variation of the superconducting energy gap and provide evidence for a pseudogap state where the gap persists well above the superconducting transition temperature (T C ) up to a characteristic temperature T* in the strong disorder limit. This result is supported by magnetoresistance (MR) measurements where we observe that a strong positive MR persists up to the pseudogap temperature T*.