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TT: Fachverband Tiefe Temperaturen

TT 85: Superconductivity: Yu-Shiba-Rusinov and Andreev Physics

TT 85.1: Talk

Thursday, March 12, 2026, 15:00–15:15, CHE/0089

Tip-gated quantum phase transition via valence change in a single Yu-Shiba-Rusinov impurity — •Xingsen Chen¹, Junyi Zhang², and Hao Zheng¹ — ¹Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai, China — ²Johns Hopkins University, Baltimore, USA

We report the real-space visualization of a quantum phase transition driven by a valence change in a single magnetic impurity coupled to a superconductor. Using a scanning tunneling microscope on a proximitized Bi₂Te₃/NbSe₂ substrate, we employ tip-induced band bending to precisely gate the energy level of a single Fe-based impurity cluster across the Fermi energy.

This tuning induces the formation of Yu-Shiba-Rusinov states inside the superconducting gap. Their evolution and zero-energy crossing signal a quantum phase transition. By correlating the YSR state transition with a concurrent high-energy valence transition, we identify the mechanism as a single-electron valence change of the Fe impurity (Fe²⁺/Fe³⁺), rather than conventional Kondo screening. Theoretical modelling using a multi-orbital Anderson impurity model confirms that the transition is governed by resonant hybridization and charge fluctuations at the mixed-valence point.

Our work establishes tip-gating as a powerful method for in-situ control of atomic-scale quantum phases and reveals a distinct mechanism for quantum phase transitions in superconducting hybrids.

Keywords: Yu-Shiba-Rusinov states; Quantum phase transition; Scanning tunneling microscopy; Tip-induced band bending; Single impurity