Vortex core properties in iron pnictides

Abstract

The mechanism of unconventional superconductivity in recently discovered Fe-based superconductors has been intensively discussed. A plausible candidate is the superconducting (SC) pairing mediated by antiferromagnetic (AFM) interactions. There are two different approaches predicting the s± pairing state, in which the SC gap shows an s-wave symmetry that changes sign between different Fermi-surface (FS) sheets. The first one is based on the itinerant spin fluctuations promoted by FS nesting, and the second is based on the local AFM exchange couplings. We apply quasiclassical Eilenberger approach to the vortex state to calculate the cutoff parameter, ξh, at different levels of impurity scattering rates and to compare results with experimental data for iron pnictides. The s±-wave pairing symmetry is considered as a presumable state for these materials. Magnetic field dependence of ξh/ξc2 is found to be nonuniversal for s± pairing: depending on the chosen parameter set it can reside both below and above analytical Ginzburg-Landau curve. It is also found that normalized ξ2/ξc2(B/Bc2) dependence is increasing with pair-breaking (interband) impurity scattering, and the intraband scattering results in decreasing of the ξ2/ξc2 value. Here, ξ2 is the vortex core size and ξc2 is the Ginzburg-Landau coherence length determined from the upper critical field. The ξ2/ξc2(B/Bc2) curve has a minimum at low temperatures and small scattering evolving into monotonously decreasing function at strong scattering and high temperatures.