Anisotropy and spin-fluctuation effects on the spectral properties of Shiba impurities

TL;DR

This paper models the effects of spin-fluctuations and magnetic anisotropy on the spectral properties of Shiba states in superconductor-impurity systems, emphasizing the importance of quantum effects for interpreting STM experiments.

Contribution

It introduces a quantum theoretical framework that includes spin-fluctuations and anisotropy, improving upon classical-spin models for Shiba states.

Findings

Quantum fluctuations significantly alter Shiba state energies.

Magnetic anisotropy impacts the subgap spectral features.

Results aid in better interpreting STM measurements.

Abstract

We theoretically consider a quantum magnetic impurity coupled to a superconductor, and obtain the local density of states at the position of the impurity taking into account the effect of spin-fluctuations and single-ion magnetic anisotropy. We particularly focus on the spectrum of subgap Yu-Shiba-Rusinov (YSR or Shiba) states induced by a quantum impurity with easy- or hard-axis uniaxial anisotropy. Although this is a relevant experimental situation in, e.g., magnetic adatoms on the surface of clean metals, it is customary that theoretical descriptions assume a classical-spin approximation which is not able to account for single-ion anisotropy and other quantum effects. Here, quantum fluctuations of the spin are taken into account in the equations of motion of the electronic Green's function in the weak-coupling limit, and considerably modify the energy of the Shiba states compared to the classical-spin approximation. Our results point towards the importance of incorporating quantum fluctuations and anisotropy effects for the correct interpretation of scanning tunneling microscopy (STM) experiments.