Schönenberger, Christian

Date:   Thursday, December 17, 2020
Time:   11:00
Place:   scheduled Zoom meeting
Host:    Klaus Ensslin

Superconducting proximity effect in semiconducting nanowires: from the Andreev bound state to Andreev molecule

Christian Schönenberger
University of Basel

C. Jünger¹, S. F. Thomas¹, R. Delagrange^1,2, D. Chevallier¹, L. Gubser¹, G. Fülöp^1,3, M. Nilsson¹, S. Lehmann⁴, K.A. Dick⁴, C. Thelander⁴, F. Rossi⁵, V. Zannier⁵, L. Sorba⁵, J. Klinovaja¹, D. Loss¹, A. Baumgartner¹ and C. Schönenberger¹

1 Department of Physics and Swiss Nanoscience Institute (SNI), University of Basel, CH-4056 Basel, Switzerland
2 Service de Physique de l'Etat Condensé, CEA, Spec, L'orme des merisiers 91191 Cedex, Gif-sur-Yvette, France
3 Budapest University of Technology and Economics, Dep. of Physics. Budapest, Hungary
4 Divison of Solid State Physics and NanoLund, Lund University, S-221 00 Lund, Sweden and Center for Analysis and Synthesis, Lund University, S-221 00 Lund, Sweden
5 Istituto Nanoscienze - CNR and Scuola Normale Superiore, 56127 Pisa, Italy and IMEM—CNR, Parco Area delle Scienze, I-43124 Parma, Italy

Sub-gap states in semiconducting-superconducting nanowire (NW) hybrid devices are currently controversially discussed. One source of ambiguity is the lack of an energetically and spatially well-defined tunnel spectrometer. Here, we use quantum dots (QDs) directly integrated into the NW during the growth process to perform tunnel spectroscopy of discrete sub-gap states in a NW segment that terminates on one side at a superconducting contact (S), realizing an S-NW-QD-NW-N device [1]. 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. We further explore the magnetic-field dependence of the sub-gap states, which we think are ABS appearing in the lead [2]. To our surprise, we also found sup-gate states at finite energy that do not disperse in magnetic field. This is explained by a large spin-orbit interaction that effectively freezes out the Zeeman splitting due to the formation of a “stiff” spatial spin texture. We also analyze S-NW-QD-NW-S devices where proximity induced superconductivity can be induced at both sides. The respective ABSs can hybridize with a QD state. This can further lead to a coupling between the ABS states on the left and right side mediated by the QD. We argue that this hybridization is an Andreev molecule state, a precursor to a Josephson junction that mediates the Josephson coupling. Different to QDs where the barriers are formed electrostatically by gates, the in-situ grown QDs in NWs are exceptionally stable.[3] Finite-bias Coulomb blockade measurements can be taken over many charge states (up to ~ 100) in a reproducible manner. The spectra show rich physics of lead states that form within the NW segment between the QD and the metallic contacts. We will also outline our current understanding of these lead states.

[1] C. Jünger et al., Communications Physics 2, 76 (2019)
[2] C. Jünger et al., Phys. Rev. Lett.(accepted), arXiv:2001.07666 (2020)
[3] F. Thomas et al., Nanotechnology 31, 7 (2020)