Finite Length Effects and Coulomb Interaction in Ge Quantum Well-Based Josephson Junctions Probed with Microwave Spectroscopy

TL;DR

This study uses microwave spectroscopy to explore Andreev bound states in Ge quantum well Josephson junctions, revealing the effects of finite length and Coulomb interactions on their spectra.

Contribution

It introduces models that incorporate finite-length effects and Coulomb interactions to explain observed ABS spectra in Ge-based Josephson junctions.

Findings

Multiple ABSs observed in 350 nm junctions

Finite-length effects influence ABS spectra

Coulomb interactions are significant in 1.2 μm junctions

Abstract

Proximitized Ge quantum wells have emerged as a novel platform for studying Andreev bound states (ABSs), due to their expected strong spin-orbit interaction and high mobility. Here, we used microwave spectroscopy techniques to investigate ABSs in Josephson junctions (JJs) realized in proximitized Ge quantum wells. Spectroscopic signatures observed in a 350 nm junction indicated the presence of multiple ABSs, and were reproduced with a model including finite-length effects. The ABS spectra measured for a $1.2~\mu$m junction were explained by a model including three ABSs in two conduction channels and finite Coulomb interaction. Our work highlights the importance of interactions in JJs and serves as a basis for understanding and manipulating ABSs in Ge-based hybrid devices.