Pairing symmetry in a two-orbital Hubbard model on a square lattice

TL;DR

This paper explores unconventional superconducting pairing symmetries in a two-orbital Hubbard model on a square lattice, revealing novel spin-triplet and spin-singlet states enabled by multi-orbital effects and finite momentum pairing.

Contribution

It demonstrates the emergence of s-wave spin-triplet and p-wave spin-singlet pairing states in a multi-orbital system, which are forbidden in single-orbital models, and discusses finite momentum pairing without magnetic fields.

Findings

Identification of s-wave spin-triplet orbital-antisymmetric state

Discovery of p-wave spin-singlet orbital-antisymmetric state

Finite momentum pairing states in multi-Fermi-surface systems

Abstract

We investigate superconductivity in a two-orbital Hubbard model on a square lattice by applying fluctuation exchange approximation. In the present model, the symmetry of the two orbitals are assumed to be that of an s orbital. Then, we find that an s-wave spin-triplet orbital-antisymmetric state and a p-wave spin-singlet orbital-antisymmetric state appear when Hund's rule coupling is large. These states are prohibited in a single-orbital model within states with even frequency dependence, but allowed for multi-orbital systems. We also discuss pairing symmetry in other models which are equivalent to the two-orbital Hubbard model except for symmetry of orbitals. Finally, we show that pairing states with a finite total momentum, even without a magnetic field, are possible in a system with two Fermi-surfaces.