Superconductivity in a Two-Orbital Hubbard Model with Electron and Hole Fermi Pockets: Application in Iron Oxypnictide Superconductors

TL;DR

This study uses an exact diagonalization approach to explore superconductivity in a one-dimensional two-orbital Hubbard model with electron and hole pockets, revealing two distinct pairing phases relevant to iron oxypnictide superconductors.

Contribution

It identifies two types of superconducting phases and clarifies their pairing mechanisms, connecting theoretical models to experimental observations in iron-based superconductors.

Findings

Two superconducting phases identified: SC u'-large and SC u-large.

Dominant pairing is intersite spin-singlet for SC u'-large and on-site for SC u-large.

Results support sign-reversing s-wave pairing in iron oxypnictide superconductors.

Abstract

We investigate the electronic states of a one-dimensional two-orbital Hubbard model with band splitting by the exact diagonalization method. The Luttinger liquid parameter $K_{\rho}$ is calculated to obtain superconducting (SC) phase diagram as a function of on-site interactions: the intra- and inter-orbital Coulomb $U$ and $U'$, the Hund coupling $J$, and the pair transfer $J'$. In this model, electron and hole Fermi pockets are produced when the Fermi level crosses both the upper and lower orbital bands. We find that the system shows two types of SC phases, the SC \Roman{u'-large} for $U>U'$ and the SC \Roman{u-large} for $U<U'$, in the wide parameter region including both weak and strong correlation regimes. Pairing correlation functions indicate that the most dominant pairing for the SC \Roman{u'-large} (SC \Roman{u-large}) is the intersite (on-site) intraorbital spin-singlet with (without) sign reversal of the order parameters between two Fermi pockets. The result of the SC \Roman{u'-large} is consistent with the sign-reversing s-wave pairing that has recently been proposed for iron oxypnictide superconductors.