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Andreev-Enhanced Conductance Quantization and Gate-Tunable Induced Superconducting Gap in Germanium

Elyjah Kiyooka1,*,‡, Chotivut Tangchingchai1, Gonzalo Troncoso Fernandez-Bada1, Boris Brun-Barriere1, Simon Zihlmann1, Romain Maurand1, Francois Lefloch1, Vivien Schmitt1, Jean-Michel Hartmann2 et al.

Manuel Houzet1 and Silvano De Franceschi1,†

  • *Contact author: elyjah.kiyooka@polytechnique.edu
  • †Contact author: silvano.defranceschi@cea.fr
  • ‡Present address: QCMX Lab, Laboratoire de Physique de la Matière condensée, CNRS, École polytechnique, Institut Polytechnique de Paris, 91120 Palaiseau, France.
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PRX Quantum 7, 033043 – Published 1 September, 2026

DOI: https://doi.org/10.1103/zd45-rvtk

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.

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Popular Summary

A superconductor in intimate electrical contact with a semiconductor results in a rich physical system where superconducting pairing correlations combine with single-particle effects such as gate-tunable quantum confinement and reduced dimensionality. This work investigates this phenomenology in a relatively recent hybrid system composed of a Ge/SiGe heterostructure embedding a high-mobility two-dimensional hole gas contacted by superconducting aluminum. Specifically, we address the regime of one-dimensional transport in gate-defined quantum point contacts positioned close to the semiconductor/superconductor interface. We observe the characteristic conductance quantization with steps consistently enhanced by the superconducting proximity effect, in unprecedented agreement with theoretical predictions [Beenakker, Phys. Rev. B 46, 12841 (1992)]. In addition, we operate quantum-point contacts close to pinch off to perform a tunneling spectroscopy of the superconducting gap induced in the two-dimensional hole gas. We show that the gap can be tuned by varying the hole density via the voltage applied to a top gate. The reported experimental study carries fundamental importance in the understanding of the superconducting proximity effect in an emerging semiconductor platform.

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PRX Quantum 7, 033043– Published 1 September, 2026
  • Received 1 April 2026
  • Revised 15 June 2026
  • Accepted 30 June 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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