Superconducting quantum effects in Ge-Si core-shell nanowires

Ge-Si core-shell nanowires feature one-dimensional confinement, low disorder and strong ‘first order’ Rashba spin-orbit interaction (SOI) [1]. These wires can therefore be utilised to realize various advanced applications such as an (Andreev) spin-qubit and a long-range quantum bus based on strong spin-photon coupling. Additionally, when combined with an s-wave superconductor and a sufficiently large Zeeman field - or in the case of double nanowires a sufficiently large crossed-Andreev excitation gap [2] - Ge-Si nanowires should undergo a topological phase transition that hosts Majorana fermions [3].

With the latter in mind, we use superconducting Al leads on the Ge-Si nanowire to explore the interaction between the superconducting condensate and quantum states in the wire. We use two different types of devices: (i) gate-tunable nanowire Josephson junctions and (ii) tunnel spectroscopy devices which combine a superconducting contact with normal contacts and tunneling gates. A novel contacting recipe gives us control over the tunnel coupling between superconductor and semiconductor and therefore prevents renormalization of the SOI and g-factor towards the bulk Al value; an essential step towards realizing a topological phase transition. Furthermore, we explore a redistribution of Shapiro step patterns and an anomalous enhancement of the critical Josephson current, both as a function of magnetic field.

[1] PRB 90, 155437 (2014)

[2] PRB 90, 195421 (2014)

[3] PRB 97, 045415 (2018)