Perturbation Theory of High-Tc Superconductivity in Iron Pnictides

TL;DR

This paper uses perturbation theory on multi-band Hubbard models to analyze high-Tc superconductivity in iron pnictides, suggesting an unconventional s_{+-}-wave pairing mechanism consistent with observed high transition temperatures.

Contribution

It demonstrates that a weak-coupling perturbative approach with realistic parameters can explain high Tc and proposes an s_{+-}-wave pairing symmetry in iron pnictides.

Findings

Large eigenvalues of the Eliashberg equation explain high Tc.

Unconventional, non-phonon-mediated superconductivity is likely.

The pairing symmetry is an s_{+-}-wave, nodeless on Fermi surfaces.

Abstract

The high-transition-temperature (high-Tc) superconductivity discovered recently in iron pnictides is analyzed within a perturbation theory. Specifically, the probable pairing symmetry, the doping dependence of the transition temperature and the pairing mechanism are studied by solving the Eliashberg equation for multi-band (2- and 5-band) Hubbard models with realistic electronic structures. The effective pairing interaction is expanded perturbatively in the on-site Coulomb integrals up to third order. Our perturbative weak-coupling approach shows that sufficiently large eigenvalues of the Eliashberg equation are obtained to explain the actual high transition temperatures by taking realistic on-site Coulomb integrals in the 5-band model. Thus, unconventional (non-phonon-mediated) superconductivity is highly likely to be realized. The superconducting order parameter does not change its sign on the Fermi surfaces, but it does change between the electron and hole Fermi surfaces. Consequently, the probable pairing symmetry is always "a nodeless extended s-wave symmetry (more specifically, an s_{+-}-wave symmetry)" over the whole parameter region that we investigated. It is suggested that the 2-band model is insufficient to explain the high values of Tc.