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

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.

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