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 systems where strong isotropic electron-phonon coupling drives the on-site Hubbard repulsion U to negative while leaving the exchange term unaffected. We solve the model at zero temperature and half filling using dynamical mean-field theory, focusing on the intraorbital singlet superconducting phase and discarding other possible instabilities, such as interorbital pairing and charge-density wave ordering. Already at J=0, the two-orbital model features a superconductor-insulator transition as |U| grows, in contrast 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, even more significantly, in the superconducting state. However, this pushes the system towards an effectively stronger coupling and hence to a faster transition to the insulating state. Similar to the Mott transition in the repulsive model, the superconductor-insulator transition here is marked by a vanishing quasiparticle weight Z. This leads to a scenario that recalls strongly correlated superconductivity close to a Mott transition, where pairing is enhanced but phase coherence is rapidly lost, even though the present model is dominated by attractive interactions.

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