Superconductor-Insulator transition in a two-orbital attractive Hubbard model with Hund's exchange

TL;DR

This paper investigates a two-orbital attractive Hubbard model with Hund's exchange, revealing a superconductor-insulator transition driven by interaction strength, with implications for strongly correlated superconductivity near a Mott transition.

Contribution

It introduces a detailed analysis of the superconductor-insulator transition in a two-orbital model with Hund's exchange, highlighting the effects of finite exchange on the transition.

Findings

Superconductor-insulator transition occurs at finite attractive interaction strength.

Hund's exchange enhances the effective coupling, accelerating the transition.

Transition is characterized by vanishing quasi-particle weight, resembling Mott physics.

Abstract

We study a two-orbital attractive Hubbard model with a repulsive Hund's exchange coupling $J$ as an idealized model for a two-band superconductor. This framework is motivated by a system where a large isotropic electron-phonon coupling drives the on-site Hubbard repulsion $U$ negative, while leaving the exchange term unaffected. We focus on the intra-orbital (intra-band) singlet superconducting phase and we solve the model at zero temperature and half-filling using Dynamical Mean-Field Theory. Already at $J=0$, the two-orbital model features a superconductor-insulator transition as $\vert U\vert$ grows, as opposed to the single-orbital case which remains superconducting for any $U < 0$. We find that a finite $J$ strengthens the effect of the attractive $U$, both in the normal state and, more significantly, in the superconducting state. However, this pushes the system towards an effectively stronger coupling, hence to a faster transition to the insulating state. Remarkably, the transition from a superconductor to an insulator occurs with a vanishing quasi-particle weight $Z$ and in a scenario that recalls strongly correlated superconductivity close to a Mott transition, even though the present model is dominated by attractive interactions.