# Hard superconducting gap and diffusion-induced superconductors in Ge-Si nanowires

## TL;DR

This study demonstrates a hard induced superconducting gap in Ge-Si nanowire Josephson transistors, achieved through annealing that causes aluminium diffusion, enabling potential Majorana mode detection and high-field applications.

## Contribution

The paper introduces a novel annealing process that creates diffusion-induced superconductors within Ge-Si nanowires, enhancing their superconducting properties and magnetic field resilience.

## Key findings

- Hard superconducting gap observed up to 250 mT
- Diffusion-induced superconductors exhibit distinct critical temperatures and fields
- Selective switching of superconductors enables control over supercurrent

## Abstract

We show a hard induced superconducting gap in a Ge-Si nanowire Josephson transistor up to in-plane magnetic fields of $250$ mT, an important step towards creating and detecting Majorana zero modes in this system. A hard induced gap requires a highly homogeneous tunneling heterointerface between the superconducting contacts and the semiconducting nanowire. This is realized by annealing devices at $180$ $^\circ$C during which aluminium inter-diffuses and replaces the germanium in a section of the nanowire. Next to Al, we find a superconductor with lower critical temperature ($T_\mathrm{C}=0.9$ K) and a higher critical field ($B_\mathrm{C}=0.9-1.2$ T). We can therefore selectively switch either superconductor to the normal state by tuning the temperature and the magnetic field and observe that the additional superconductor induces a proximity supercurrent in the semiconducting part of the nanowire even when the Al is in the normal state. In another device where the diffusion of Al rendered the nanowire completely metallic, a superconductor with a much higher critical temperature ($T_\mathrm{C}=2.9$ K) and critical field ($B_\mathrm{C}=3.4$ T) is found. The small size of diffusion-induced superconductors inside nanowires may be of special interest for applications requiring high magnetic fields in arbitrary direction.