Bound states in superconductors revealed by Josephson tunneling
Final Report Abstract
Magnetic adsorbates on superconducting substrates have recently attracted immense interest as building blocks of topological adatom chains, which constitute a promising platform for quantum computation. The exchange interaction of magnetic atoms with a superconducting substrate gives rise to so-called Yu-Shiba-Rusinov (YSR) states inside the superconducting energy gap. While these appear as localized states for single magnetic atoms, they can form extended YSR bands when hybridized in nanostructures, such as adatom chains. This can eventually lead to the emergence of topological superconductivity. The unambiguous identification of topological properties requires a toolbox going beyond standard scanning tunneling microscopy/spectroscopy (STM/STS) experiments. Our DFG-ANR project explored new aspects of quantum transport phenomena in tunnel junctions including magnetic adatoms and quantum dots. The complementary transport geometries together with new experimental advances allowed us to investigate transport in different regimes, from tunneling to contact, in magnetic field and under microwave irradiation. We first used a quantum-transport set-up to investigate and tune the properties of a quantum dot coupled to two leads. We showed that the YSR states are tunable by an external magnetic field, allowing the states to cross the quantum phase transition between being screened and unscreened (published in Phys. Rev. Res.). The second type of experiments were done in STM geometry, where we combined conventional differential conductance spectroscopy with microwave irradiation. This allowed us to detect photon-assisted tunneling. The signatures in power-dependent photon-assisted tunneling maps allowed us to draw unambiguous conclusions on the transport properties and in particular the conductancedependent transition from single-electron tunneling to resonant Andreev reflections. By applying the well-known Tien-Gordon model to tunneling rates, we reproduced the experimental data with very high accuracy and established photon-assisted tunneling as a powerful tool to analyze tunneling processes (published in Nature Physics). We further established current-driven Josephson spectroscopy as a complementary tool to the common voltage-driven regime in an STM geometry. We could thus determine the switching and retrapping currents separating the dissipationless Josephson supercurrent from the dissipative current flow. When magnetic adatoms were inserted in the junctions, we observed non-reciprocal behavior, reminiscent of a superconducting diode (accepted for publication in Nature). Our project thus established novel experimental protocols, unifying surface science and quantum nanoelectronics, which can now be applied for the identification of novel types of superconductivity on the nanoscale. This project allowed to build up a close collaboration between the groups of Katharina Franke at Freie Universität Berlin and Clemens Winkelmann at CNRS Délégation Alpes / Institut Néel in Grenoble, which is to be continued in the future.
Publications
- Magnetic-Field-Induced Transition in a Quantum Dot Coupled to a Superconductor. Physical Review Research 2, 012065 (R) (2020)
Alvaro Garcia-Corral, David M. T. van Zanten, Katharina J. Franke, Hervé Courtois, Serge Florens, Clemens B. Winkelmann
(See online at https://doi.org/10.1103/PhysRevResearch.2.012065) - Resonant Andreev reflections probed by photon-assisted tunnelling at the atomic scale. Nature Physics 16, 1222 (2020)
Olof Peters, Nils Bogdanoff, Sergio Acero Gonzalez, Larissa Melischek, J. Rika Simon, Gaël Reecht, Clemens B. Winkelmann, Felix von Oppen, Katharina J. Franke
(See online at https://doi.org/10.1038/s41567-020-0972-z) - Direct observation of intrinsic surface magnetic disorder in amorphous superconducting films. Physical Review B 105, L140505 (2022)
Idan Tamir, Martina Trahms, Franzisca Gorniaczyk, Felix von Oppen, Dan Shahar, Katharina J. Franke
(See online at https://doi.org/10.1103/PhysRevB.105.L140505) - Diode effect in Josephson junctions with a single magnetic atom. Nature 615, 628 (2023)
Martina Trahms, Larissa Melischek, Jacob F. Steiner, Bharti Mahendru, Idan Tamir, Nils Bogdanoff, Olof Peters, Gael Reecht, Clemens B. Winkelmann, Felix von Oppen, Katharina J. Franke
(See online at https://doi.org/10.1038/s41586-023-05743-z)
