Bound States of Defects in Superconducting LiFeAs Studied by Scanning Tunneling Spectroscopy

TL;DR

This study uses scanning tunneling microscopy and spectroscopy to analyze defect states in LiFeAs, revealing bound states and symmetry-breaking effects consistent with s± gap symmetry in this superconductor.

Contribution

It provides detailed real-space imaging and spectroscopic analysis of defect-induced bound states in LiFeAs, supporting the s± pairing symmetry model.

Findings

Identified defect types and their lattice positions.

Observed bound states near the gap edges and within the gaps.

Detected complex, possibly chiral, LDOS patterns around defects.

Abstract

Defects in LiFeAs are studied by scanning tunneling microscopy (STM) and spectroscopy (STS). Topographic images of the five predominant defects allow the identification of their position within the lattice. The most commonly observed defect is associated with an Fe site and does not break the local lattice symmetry, exhibiting a bound state near the edge of the smaller gap in this multi-gap superconductor. Three other common defects, including one also on an Fe site, are observed to break local lattice symmetry and are pair-breaking indicated by clear in-gap bound states, in addition to states near the smaller gap edge. STS maps reveal complex, extended real-space bound state patterns, including one with a chiral distribution of the local density of states (LDOS). The multiple bound state resonances observed within the gaps and at the inner gap edge are consistent with theoretical predictions for s$^{\pm}$ gap symmetry proposed for LiFeAs and other iron pnictides.