Andreev states in Ge-Si core-shell nanowire Josephson devices
Zhen Wu1, Joost Ridderbos1, Ang Li2, Erik P. A. M. Bakkers2, Chuan Li1, Alexander Brinkman1, and Floris A. Zwanenburg1
1 MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands
2 Department of Applied Physics, Eindhoven University of Technology, Eindhoven, The Netherlands
Ge-Si core-shell nanowires feature one-dimensional confinement, low disorder and strong ‘first order’ Rashba spin-orbit interaction (SOI) [1]. Additionally, when combined with an s-wave superconductor and a sufficiently large Zeeman field, Ge-Si nanowires should undergo a topological phase transition that hosts Majorana bound states [2]. Here, we use gate-tunable nanowire Josephson devices (Figure 1a) to explore Andreev bound states and topological transition conditions in the wire. A novel contacting recipe gives us control over the tunnel coupling between superconductor and semiconductor and contributes to the observation of a rich subgap spectra in a Josephson device (Figure 1b). Furthermore, changing the Si shell thickness gives us control over the induced superconducting gap and thus over the coupling to the superconductor. Optimizing this coupling strength prevents renormalization of the SOI and g-factor towards the bulk Al value; an essential step towards realizing a topological phase transition[3].
Figure 1 (a) Schematic overview of a gate tunable Ge-Si core-shell nanowire Josephson device. (b) Bias spectroscopy of the subgap states in the device.
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[2] Maier F, Klinovaja J, Loss D. Physical Review B, 90(19), 195421 (2014).
[3] Reeg, C., Loss, D., & Klinovaja, Physical Review B, 97(16), 165425 (2018).