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Transparency-controlled multiple charge transfer in superconducting junctions with local shot-noise scanning tunneling spectroscopy

Yudai Sato1,2,3,4, Maialen Ortego Larrazabal1,5, Jian-Feng Ge1,6, Ingmar Swart5, Doohee Cho7, Wolfgang Belzig8, Juan Carlos Cuevas9,10, Milan P. Allan1,2,3,4,*, and Jiasen Niu1,2,3,4,†

  • *Contact author: milan.allan@lmu.de
  • †Contact author: niu@physics.leidenuniv.nl
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Phys. Rev. B 114, 204508 – Published 26 October, 2026

DOI: https://doi.org/10.1103/81fz-49c6

Abstract

Charge transport in superconducting junctions at finite voltages is governed by Andreev reflections, including multiple Andreev reflections, which are processes that enable multiple charge transfer, a hallmark that shot noise can directly quantify. Since the effective charge extracted from shot-noise measurements varies with the transparency of the junction, systematic control of transparency is essential but experimentally challenging. Here, we present shot-noise scanning tunneling microscopy measurements enabled by a newly developed amplifier, allowing access to different transparency regimes. We perform shot-noise measurements on Pb(111) with tunable transparency at 2.2 K and observe that the shot noise evolves from a single-electron tunneling regime to multiple charge-transfer regimes as transparency increases. Our results are quantitatively consistent with theoretical simulations of Andreev reflections and multiple Andreev reflections for a single-channel system. These results establish junction transparency as the key parameter governing the evolution of charge transport and demonstrate that noise STM is a powerful platform for investigating microscopic charge transport mechanisms with controlled junction transparency at the atomic scale.

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Phys. Rev. B 114, 204508– Published 26 October, 2026
  • Received 23 March 2026
  • Accepted 25 September 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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