Electronic structure around a vortex core in iron pnictide superconductors

Recently bound states in vortex cores were observed by scanning tunneling microscopy measurement in iron pnictides. The local density of states ρ(ω,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_+-) pairing, and found results in qualitative agreement with the experiment. We argue that the peak energy ω_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_+- pairing.