Theory of quasiparticle vortex bound states in Fe-based superconductors: application to LiFeAs

TL;DR

This paper models vortex bound states in anisotropic superconductors, highlighting how Fermi surface effects can obscure gap anisotropy in spectroscopic measurements, with specific application to LiFeAs.

Contribution

It provides a quasiclassical calculation framework for vortex states considering both gap and Fermi surface anisotropy, applied explicitly to LiFeAs.

Findings

Fermi surface anisotropy can overshadow gap anisotropy in vortex spectroscopy.

Direct observation of gap minima or nodes via STS is challenging in anisotropic superconductors.

Caution is advised when interpreting vortex STS data in Fe-based superconductors like LiFeAs.

Abstract

Spectroscopy of vortex bound states can provide valuable information on the structure of the superconducting order parameter. Quasiparticle wavefunctions are expected to leak out in the directions of gap minima or nodes, if they exist, and scanning tunneling spectroscopy (STS) on these low-energy states should probe the momentum dependence of the gap. Anisotropy can also arise from band structure effects, however. We perform a quasiclassical calculation of the density of states of a single vortex in an anisotropic superconductor, and show that if the gap itself is not highly anisotropic, the Fermi surface anisotropy dominates, preventing direct observation of superconducting gap features. This serves as a cautionary message for the analysis of STS data on the vortex state on Fe-based superconductors, in particular LiFeAs, which we treat explicitly.