Scanning Tunneling Microscopy/Spectroscopy of Vortices in LiFeAs

TL;DR

This study uses scanning tunneling microscopy/spectroscopy to analyze vortex structures in LiFeAs, revealing fully-gapped multi-band superconductivity, vortex lattice behavior, and the Kramer-Pesch effect, advancing understanding of vortex matter in this superconductor.

Contribution

It provides the first detailed imaging and spectroscopic analysis of vortices in LiFeAs, highlighting vortex lattice structures, spectral features, and the Kramer-Pesch effect in a clean superconductor.

Findings

Vortices form a quasi-hexagonal lattice with domain boundaries at low fields.

Vortex spectra show a sharp peak below the Fermi energy, indicating particle-hole asymmetry.

Vortex cores shrink with decreasing temperature, evidencing the Kramer-Pesch effect.

Abstract

We investigate vortices in LiFeAs using scanning tunneling microscopy/spectroscopy. Zero-field tunneling spectra show two superconducting gaps without detectable spectral weight near the Fermi energy, evidencing fully-gapped multi-band superconductivity. We image vortices in a wide field range from 0.1 T to 11 T by mapping the tunneling conductance at the Fermi energy. A quasi-hexagonal vortex lattice at low field contains domain boundaries which consist of alternating vortices with unusual coordination numbers of 5 and 7. With increasing field, the domain boundaries become ill-defined, resulting in a uniformly disordered vortex matter. Tunneling spectra taken at the vortex center are characterized by a sharp peak just below the Fermi energy, apparently violating particlehole symmetry. The image of each vortex shows energy-dependent 4-fold anisotropy which may be associated with the anisotropy of the Fermi surface. The vortex radius shrinks with decreasing temperature and becomes smaller than the coherence length estimated from the upper critical field. This is direct evidence of the Kramer-Pesch effect expected in a clean superconductor.