Interorbital pairing and its physical consequences for iron pnictide superconductors

Abstract

We study interorbital pairings of iron pnictide superconductors within a minimal two-orbital tight-binding model. We find that in real space, a set of self-consistently determined pairing order parameters forms two sublattices with a relative phase of $\ensuremath{\pi}$ and the pairing symmetry is ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}\ensuremath{\sim}\text{cos}\text{ }{k}_{x}\ensuremath{-}\text{cos}\text{ }{k}_{y}$. In momentum space, it corresponds to the $\ensuremath{\eta}$ pairing proposed by Yang [Phys. Rev. Lett. 63, 2144 (1989)], with nonzero momenta of Cooper pairs. One physical consequence of this type of pairing is the existence of a significant amount of zero energy (gapless) states around the Fermi surface even in the absence of disorder, which contradicts current experiments thus excluding such a pairing in iron pnictide superconductors.