Signatures of nodeless multiband superconductivity and particle-hole crossover in the vortex cores of FeTe$_{0.55}$Se$_{0.45}$

TL;DR

This study uses a microscopic multiband model to analyze vortex core states in FeTe$_{0.55}$Se$_{0.45}$, revealing complex bound state patterns and the effects of disorder, aligning with recent experimental observations.

Contribution

It provides a detailed theoretical analysis of vortex bound states in a multiband superconductor, accounting for disorder and particle-hole crossover effects, which were previously not fully understood.

Findings

Existence of well-separated vortex bound states confirmed

Disorder does not significantly alter vortex-core electronic structure

Vortex states transition from hole-like to electron-like with energy

Abstract

Scanning tunneling experiments on single crystals of superconducting FeTe$_{0.55}$Se$_{0.45}$ have recently provided evidence for discrete energy levels inside vortices. Although predicted long ago, such levels are seldom resolved due to extrinsic (temperature, instrumentation) and intrinsic (quasiparticle scattering) limitations. We study a microscopic multiband model with parameters appropriate for FeTe$_{0.55}$Se$_{0.45}$. We confirm the existence of well-separated bound states and show that the chemical disorder due to random occupation of the chalcogen site does not affect significantly the vortex-core electronic structure. We further analyze the vortex bound states by projecting the local density of states on angular-momentum eigenstates. A rather complex pattern of bound states emerges from the multiband and mixed electron-hole nature of the normal-state carriers. The character of the vortex states changes from hole-like with negative angular momentum at low energy to electron-like with positive angular momentum at higher energy within the superconducting gap. We show that disorder in the arrangement of vortices most likely explains the differences found experimentally when comparing different vortices.