Electronic structure around a vortex core in iron pnictide superconductors

Abstract

Recently bound states in vortex cores were observed by scanning tunneling microscopy measurement in iron pnictides. The local density of states $\ensuremath{\rho}(\ensuremath{\omega},r)$ is asymmetrically peaked below the Fermi energy for $r=0$ (at the vortex core) and the peak splits and merges to the gap edges away from the vortex core $(r>0)$. We performed exact large-scale calculation of the vortex-core electronic structure in effective lattice models with both in-phase $s$-wave $({s}_{++})$ and antiphase $s$-wave $({s}_{+\ensuremath{-}})$ pairing, and found results in qualitative agreement with the experiment. We argue that the peak energy ${\ensuremath{\omega}}_{p}$ is determined by the normal-state band structure, insensitive to the relative phase of the pairing gaps on the multiple bands. The observed bound state is compatible, although not exclusively, with ${s}_{+\ensuremath{-}}$ pairing.