Andreev-enhanced conductance quantization and gate-tunable induced superconducting gap in germanium

TL;DR

This study demonstrates conductance quantization and gate-tunable superconducting gaps in germanium-based quantum well heterostructures, highlighting Andreev reflection effects and interface transparency in superconductor-semiconductor devices.

Contribution

It provides experimental evidence of an induced superconducting gap in germanium 2DHG with tunable properties and quantized conductance influenced by Andreev reflection.

Findings

Conductance quantization with at least four clear plateaus observed.

Andreev reflection enhances conductance steps by 40%.

Superconducting gap magnitude is tunable via gate voltage.

Abstract

Ge/SiGe quantum well heterostructures confining a high-mobility two-dimensional hole gas (2DHG) have emerged as a compelling platform for hybrid superconductor(S)-semiconductor(Sm) quantum devices. Here, we investigate the low-temperature transport properties of split-gate quantum point contacts (QPC) defined in one such heterostructure and positioned at different distances from an aluminum superconducting contact. We observe ballistic one-dimensional transport evidenced by conductance quantization with at least four clearly visible plateaus. Andreev reflection at the S/Sm interface induces a 40% enhancement of the conductance steps relative to the normal-state conductance staircase measured under a 100-mT out-of-plane magnetic field. This result is in excellent agreement with the theoretical expectation for an interface transparency of 0.88. By operating the QPCs in the tunneling regime, we probe the local density of states of the proximitized 2DHG. We report direct experimental evidence of an induced superconducting gap, demonstrating that its magnitude can be tuned by a gate voltage acting on the carrier density in the 2DHG.