Strong-coupling theory of superconductivity in a degenerate Hubbard model

TL;DR

This paper develops a strong-coupling theory for superconductivity in orbital degenerate systems using the FLEX approximation, revealing how orbital splitting influences the emergence of d-wave superconductivity and phase transitions.

Contribution

It introduces a microscopic Hamiltonian and applies the FLEX approximation to show how orbital splitting controls superconducting and magnetic phases.

Findings

d_{x^2-y^2}-wave superconductivity is induced by increasing orbital splitting

Orbital splitting energy governs transition from paramagnetic to antiferromagnetic phase

Spin and orbital fluctuations are self-consistently determined in the model

Abstract

In order to discuss superconductivity in orbital degenerate systems, a microscopic Hamiltonian is introduced. Based on the degenerate model, a strong-coupling theory of superconductivity is developed within the fluctuation exchange (FLEX) approximation where spin and orbital fluctuations, spectra of electron, and superconducting gap function are self-consistently determined. Applying the FLEX approximation to the orbital degenerate model, it is shown that the $d_{x^2-y^2}$-wave superconducting phase is induced by increasing the orbital splitting energy which leads to the development and suppression of the spin and orbital fluctuations, respectively. It is proposed that the orbital splitting energy is a controlling parameter changing from the paramagnetic to the antiferromagnetic phase with the $d_{x^2-y^2}$-wave superconducting phase in between.