Observation of Lattice and Andreev Bound States of Vortices in Ba0.6K0.4Fe2As2 Single Crystals with Scanning Tunneling Microscopy/Spectroscopy

TL;DR

This study uses scanning tunneling microscopy to observe vortex lattice structures and Andreev bound states in hole-doped Ba0.6K0.4Fe2As2, revealing spatial evolution of vortex core spectra and differences from electron-doped variants.

Contribution

First observation of a triangular vortex lattice and Andreev bound states in hole-doped BaFe2As2 using STM, highlighting differences from electron-doped counterparts.

Findings

Triangular vortex lattice observed in Ba0.6K0.4Fe2As2

Andreev bound states detected within vortex cores

Distinct spatial evolution of vortex core spectra

Abstract

For a type-II superconductor, when the applied magnetic field is higher than the lower critical value Hc1, the magnetic flux will penetrate into the superconductor and form quantized vortices, which usually are arranged in an Abrikosov lattice. For the newly discovered iron pnictide superconductors, previous measurements have shown that, in electron-doped BaFe2As2, the vortices form a highly disordered structure. In addition, the density of states (DOS) within the vortex cores1 do not exhibit the Andreev bound states in conventional superconductors. In this Letter, we report the observation of a triangular vortex lattice and the Andreev bound states in hole-doped BaFe2As2 by using a low temperature scanning tunneling microscope (STM). Detailed study of the vortex cores reveals that the spectrum of the Andreev bound states inside the vortex core exhibits a distinct spatial evolution: at the center of the vortex core, it appears as a single peak at 0.5 mV below the Fermi-energy; away from the core center, it gradually evolves into two sub-peaks and they eventually fade out. The drastic differences between the vortex cores of the electron-doped and hole-doped counterparts are illusive to the pairing mechanism of the iron pnictide superconductors.