Zeeman effects on Yu-Shiba-Rusinov states

TL;DR

This paper demonstrates a method using the Zeeman effect in ultra-low temperature scanning tunneling spectroscopy to distinguish between screened and free impurity spins in a superconductor, clarifying the nature of Yu-Shiba-Rusinov states.

Contribution

It introduces an unambiguous experimental technique to identify the ground state of magnetic impurities in superconductors via Zeeman splitting of YSR states.

Findings

YSR peak splits or shifts in magnetic field depending on impurity state

Zeeman effect distinguishes screened from free spins

Provides insights into magnetism and superconductivity interplay

Abstract

When the exchange interaction between the impurity spin and the spins of itinerant quasiparticles are strong or weak enough, the ground states for a magnetic impurity in a superconductor are the screened or free spins, respectively. In both cases, the lowest excited state is a bound state within the superconducting gap, known as the Yu-Shiba-Rusinov (YSR) state. The YSR state is spatially localized, energetically isolated, and fully spin-polarized, leading to applications such as functional scanning probes. While any application demands identifying whether the impurity spin is screened or free, a suitable experimental technique has been elusive. Here we demonstrate an unambiguous way to determine the impurity ground state using the Zeeman effect. We performed ultra-low temperature scanning tunneling spectroscopy of junctions formed between a Cu(111) surface and superconducting Nb tips decorated by single magnetic Fe atoms. Depending on the condition of the Fe adsorbate, the YSR peak in the spectrum either splits or shifts in a magnetic field, signifying that the Fe spin is screened or free, respectively. Our observations provide renewed insights into the competition between magnetism and superconductivity and constitute a basis for the applications of the YSR state.