High-Resolution Spectroscopy of the Intermediate Impurity States near a Quantum Phase Transition

TL;DR

This study explores the detailed spectroscopic evolution of Yu-Shiba-Rusinov states near a quantum phase transition in a superconductor, revealing magnetic anisotropy effects and multiple quantum states relevant for future electronics.

Contribution

It provides the first high-resolution spectroscopic analysis of multiple YSR states near a quantum phase transition, highlighting magnetic anisotropy-induced splitting.

Findings

YSR states evolve into two pairs with distinct spectral features

Higher-energy YSR branch remains rigid during the transition

Spectral weight shows exponential dependence on exchange coupling

Abstract

The intermediate behavior near a quantum phase transition is crucial for understanding the quantum criticality of various competing phases and their separate origins, yet remains unexplored for the multiple Yu-Shiba-Rusinov (YSR) states. Here, we investigated the detailed spectroscopic change of the exchange coupling-dependent YSR states near a quantum phase transition. The initially developed one pair of YSR states, induced by the Fe vacancy in monolayer Fe(Te,Se) superconductor, are clearly resolved with high resolution showing an evolution into two pairs of YSR peaks yet with dichotomy in their spectral features. Interestingly, while the lower-energy YSR branch enters the quantum phase transition region, the higher-lying one remains rigidly away from the lower-energy counterpart with a constant energy difference. Spectral weight analysis of the higher-energy branch yields an exponential dependence on the exchange coupling, which can be well rationalized by taking the two pairs of YSR states as a result of field splitting by the magnetic anisotropy. Our results unveil the intermediate region of a quantum phase transition with a magnetic anisotropy-induced splitting of the YSR resonance, and highlight a prospect for developing functional electronics based on the flexibly controllable multiple quantum states.