Tunable superconductivity in planar Germanium

III-V materials, like InAs and InSb, are the leading platforms for the realization of gatemons and for the quest of Majorana zero modes. Here we investigate Ge as an alternative platform [1]. In the past years, dramatic progress has been made in inducing superconductivity in planar germanium either by enhancing the proximity effect using a double layer of superconductors [2] or by creating a low-disorder interface between the germanium hole gas and an annealed germanosilicide superconductor [3]. Here, we show that we can reliably induce superconductivity in a germanium hole gas by evaporating aluminum on top of a thin Si_0.3Ge_0.7 spacer which separates the superconductor from the quantum well. We estimate transparencies close to unity and we reveal a superconducting hard gap which can be tuned by the Si_0.3Ge_0.7 spacer. Finally, we show the exchange of pairs of Cooper pairs between two superconducting leads, highlighting the potential of Germanium quantum well as protected qubit [4].

[1] G. Scappucci et al., Nature Reviews Materials 6.10 (2021): 926-943.

[2] K. Aggarwal et al., Physical Review Research 3.2 (2021): L022005.

[3] A. Tosato et al., arXiv:2206.00569 (2022).

[4] C. Schrade, et al., PRX Quantum 3.3 (2022): 030303.