Scanning tunneling microscopy/spectroscopy of vortices in LiFeAs

Abstract

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