Pairing mechanism for nodal s-wave superconductivity in BaFe2(As,P)2: Analysis beyond Migdal-Eliashberg formalism

Abstract

The pairing mechanism and gap structure in Ba122 pnictides have been hotly discussed for a long time as one of the central issues in Fe-based superconductors. Here, we attack this problem by taking account of the vertex corrections (VCs) for the Coulomb interaction U(U−VCs), which are totally dropped in conventional Migdal-Eliashberg formalism. The U−VC in the charge susceptibility induces strong orbital fluctuations, and the U−VC enlarges the orbital-fluctuation-driven attractive interaction. By analyzing the effective multiorbital Hubbard model for Ba122 pnictides, we find that the orbital fluctuations develop in all four d orbitals (t2g and z2 orbitals) by which the Fermi surfaces (FSs) are composed. For this reason, nearly isotropic gap functions appear on all the hole-type FSs, including the outer hole FS around the Z point composed of the z2 orbital. In contrast, nodal gap structure appears on the electron FSs for the wide parameter range. The obtained nodal s-wave state changes to a fully gapped s-wave state without sign reversal (s++-wave state) by introducing a small amount of impurities, accompanied by a small reduction in Tc. The present microscopic theory naturally explains the important characteristics of the gap structure of both hole and electron FSs in Ba122 pnictides without introducing any phenomenological pairing interaction.

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