In superconductor-semiconductor hybrid structures, superconductivity and spin polarization are competing effects because magnetic fields break Cooper pairs. They can be combined using thin films and in-plane magnetic fields, an approach that enabled the pursuit of Majorana zero modes, Kitaev chains, and Andreev spin qubits (ASQs), but remains challenging for materials with small in-plane g-factors. Here we show that granular aluminum (grAl), composed of nanometer-scale aluminum grains embedded in an amorphous oxide matrix, can overcome this limitation. By depositing grAl on Ge/SiGe heterostructures, we induce a hard superconducting gap with BCS peaks at 305 μeV and magnetic-field resilience for both the in-plane and out-of-plane directions, allowing Zeeman splitting of Yu-Shiba-Rusinov (YSR) states beyond 50 μeV (12 GHz). Leveraging this robustness, we reveal signatures of hole physics and demonstrate g-tensor tunability.
@article{arxiv.2602.21364,
title = {Granular aluminum induced superconductivity in germanium for hole spin-based hybrid devices},
author = {Giorgio Fabris and Paul Falthansl-Scheinecker and Devashish Shah and Daniel Michel Pino and Maksim Borovkov and Anton Bubis and Kevin Roux and Dina Sokolova and Alejandro Andres Juanes and Tommaso Costanzo and Inas Taha and Aziz Genç and Jordi Arbiol and Stefano Calcaterra and Afonso De Cerdeira Oliveira and Daniel Chrastina and Giovanni Isella and Ruben Seoane Souto and Maria Jose Calderon and Ramon Aguado and Jose Carlos Abadillo-Uriel and Georgios Katsaros},
journal= {arXiv preprint arXiv:2602.21364},
year = {2026}
}