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

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.