Flux-tunable Andreev bound states in hybrid full-shell nanowires

 · Invited

Abstract

Understanding excitations of the Cooper pair condensate in a superconductor is crucial for many applications in quantum information processing. A remarkable example is the possibility of creating topologically-protected non-local qubits based on quasiparticle excitations at no energy cost, so-called Majorana zero modes. Their unambiguous detection has been impeded by the ubiquitous presence of non-topological Andreev bound states pinned to zero energy. It has thus become of utmost importance to find ways to experimentally establish the physical origin of subgap states in a controlled way. Here we show that the magnetic flux tunability of full-shell nanowires allows to clearly identify subgap levels as Andreev bound states. Transport spectroscopy reveals them as Yu-Shiba-Rusinov bound states, resulting from a quantum spin impurity --a quantum dot forming within the tunneling region-- that forms Kondo-like singlets with quasiparticles in the superconductor. The magnetic flux, both through the Little-Parks modulation of the gap and the Zeeman effect, induces quantum phase transitions between physically-different ground states, resulting in subgap level crossings at zero energy -zero bias peaks in tunneling conductance- unrelated to Majoranas. Our understanding of the complex interplay of different physical effects, fully supported by theory, offers a starting point for systematic experiments towards an unambiguous topological classification of zero modes.

*This research were made possible with the support of the NOMIS Foundation and Microsoft. It received funding from the EU’s Horizon 2020 program under the grant agreements #844511, #828948, the European Research Commission through the grant agreement HEMs-DAM # 716655, the CSIC Research Platform on Quantum Technologies PTI-001, the Spanish Ministry of Science and Innovation through Grants PGC2018-097018-B-I00, PCI2018-093026, FIS2016-80434-P, RYC-2011-09345 and the Maria de Maeztu Programme CEX2018-000805-M.

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Presenters

  • Georgios Katsaros

    • Institute of Science and Technology, Austria
    • Institute of Science and Technology Austria

Authors

  • Marco Valentini

    • Institute of Science and Technology, Austria
    • Institute of Science and Technology Austria

  • Fernando Peñaranda

    • Instituto de Ciencia de Materiales de Madrid
    • CSIC - Madrid
    • CSIC-Madrid

  • Andrea Hofmann

    • Institute of Science and Technology, Austria
    • Institute of Science and Technology Austria

  • Matthias Brauns

    • Institute of Science and Technology, Austria
    • Institute of Science and Technology Austria

  • Robert Hauschild

    • Institute of Science and Technology, Austria
    • Institute of Science and Technology Austria

  • Peter Krogstrup

    • Center for Quantum Devices and Microsoft Quantum Lab Copenhagen, Niels Bohr Institute, University of Copenhagen
    • Microsoft Quantum Materials Lab and Center for Quantum Devices, Niels Bohr Institute,8University of Copenhagen, Kanalvej 7, 2800 Kongens Lyngby, Denmark
    • Niels Bohr Institute, University of Copenhagen
    • Quantum Materials Lab Copenhagen, Microsoft
    • University of Copenhagen
    • Center for Quantum Devices and Microsoft Quantum Lab Copenhagen, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark
    • Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen
    • Microsoft Quantum Materials Lab, University of Copenhagen
    • Niels Bohr Institute, Copenhagen
    • Niels Bohr Institute

  • Pablo San-Jose

    • CSIC - Madrid
    • Instituto de Ciencia de Materiales de Madrid
    • CSIC-Madrid

  • Elsa Prada

    • CSIC - Madrid
    • Instituto de Ciencia de Materiales de Madrid
    • Instituto de Ciencia de los Materiales (ICMM), Centro Superior de Investigaciones Científicas (CSIC)
    • CSIC-Madrid

  • Ramon Aguado

    • CSIC - Madrid
    • Instituto de Ciencia de Materiales de Madrid
    • CSIC-Madrid

  • Georgios Katsaros

    • Institute of Science and Technology, Austria
    • Institute of Science and Technology Austria