Pairing Symmetry in a Two-Orbital Exchange Coupling Model of Oxypnictides

TL;DR

This study investigates the pairing symmetries in a two-orbital model of oxypnictides, revealing dominant mixed s- and d-wave pairings depending on interaction parameters, with implications for understanding superconductivity in these materials.

Contribution

It systematically compares all possible pairing symmetries in a two-orbital model using mean-field calculations, identifying dominant pairing states across the phase diagram.

Findings

Mixed s_{x^2y^2} and d_{x^2-y^2} pairing symmetries are favored in large regions.

Pure s_{x^2y^2} pairing occurs when J_2 is much larger than J_1.

Opposite signs of d_{x^2-y^2} order parameters in different orbitals are observed.

Abstract

We study the pairing symmetry of a two orbital $J_1-J_2$ model for FeAs layers in oxypnictides. We vary the doping and the value of $J_1$ and $J_2$ to compare all possible pairing symmetries in a mean-field calculation. We show that the mixture of an intra-orbital unconventional $s_{x^2y^2}\sim \cos(k_x)\cos(k_y)$ pairing symmetry and a $d_{x^2-y^2}\sim \cos(k_x)-\cos(k_y)$ pairing symmetry is favored in a large part of $J_1-J_2$ phase diagram. A pure $ s_{x^2y^2}$ pairing state is favored for $J_2>>J_1$. The signs of the $d_{x^2-y^2}$ order parameters in two different orbitals are opposite. While a small $d_{xy}\sim \sin(k_x)\sin(k_y)$ inter-orbital pairing order coexists in the above phases, the intra-orbital $d_{xy}$ pairing symmetry is not favored even for large values of $J_2$.