Signatures of Amorphous Shiba State in FeTe$_{0.55}$Se$_{0.45}$

TL;DR

This study reveals that FeTe$_{0.55}$Se$_{0.45}$ hosts an amorphous state of Bogoliubov quasiparticles, originating from overlapping Shiba states, challenging the conventional understanding of its superconducting gap structure.

Contribution

It provides experimental evidence and theoretical analysis showing that the superconducting state involves overlapping Shiba states, forming an amorphous quasiparticle state in FeTe$_{0.55}$Se$_{0.45}$.

Findings

Peaks above the superconducting gap are not coherence peaks.

Overlapping Shiba states lead to an amorphous quasiparticle state.

Implications for understanding unconventional superconductivity.

Abstract

The iron-based superconductor FeTe$_{0.55}$Se$_{0.45}$ is a peculiar material: it hosts a surface state with a Dirac dispersion, is a putative topological superconductor hosting Majorana modes in vortices, and has an unusually low Fermi energy. The superconducting state is generally thought to be characterized by three gaps in different bands, with the usual homogenous, spatially extended Bogoliubov excitations -- in this work, we uncover evidence that it is instead of a very different nature. Our scanning tunneling spectroscopy data shows several peaks in the density of states above a full gap, and by analyzing the spatial and junction-resistance dependence of the peaks, we conclude that the peaks above the first one are not coherence peaks from different bands. Instead, comparisons with our simulations indicate that they originate from generalized Shiba states that are spatially overlapping. This can lead to an amorphous state of Bogoliubov quasiparticles, reminiscent of impurity bands in semiconductors. We discuss the origin and implications of this new state.