Spatially dispersing Yu-Shiba-Rusinov states in the unconventional superconductor $\mathrm{FeTe}_{0.55}\mathrm{Se}_{0.45}$

TL;DR

This study uses STM with a superconducting tip to investigate Yu-Shiba-Rusinov states in the topological superconductor $ ext{FeTe}_{0.55} ext{Se}_{0.45}$, revealing tunable impurity states influenced by electric field effects.

Contribution

It introduces a novel tip-gating approach to study impurity states in a topological superconductor, supported by a theoretical model that matches experimental observations.

Findings

Impurity states are Yu-Shiba-Rusinov type with energy tunability.

Electric field penetration affects impurity state energy.

Modeling with the single-impurity Anderson model agrees with data.

Abstract

By using scanning tunneling microscopy (STM) we find and characterize dispersive, energy-symmetric in-gap states in the iron-based superconductor $\mathrm{FeTe}_{0.55}\mathrm{Se}_{0.45}$, a material that exhibits signatures of topological superconductivity, and Majorana bound states at vortex cores or at impurity locations. We use a superconducting STM tip for enhanced energy resolution, which enables us to show that impurity states can be tuned through the Fermi level with varying tip-sample distance. We find that the impurity state is of the Yu-Shiba-Rusinov (YSR) type, and argue that the energy shift is caused by the low superfluid density in $\mathrm{FeTe}_{0.55}\mathrm{Se}_{0.45}$, which allows the electric field of the tip to slightly penetrate the sample. We model the newly introduced tip-gating scenario within the single-impurity Anderson model and find good agreement to the experimental data.