Orbitally-selective Breakdown of the Fermi Liquid and Simultaneous Enhancement of Metallic and Insulating States in Correlated Multi-band Systems with Spin-orbit Coupling

TL;DR

This paper investigates how spin-orbit coupling influences orbitally-selective Mott transitions in a three-orbital Hubbard model, revealing a novel orbitally-selective non-Fermi liquid state that explains exotic metallic behaviors in ruthenates.

Contribution

It uncovers the dual role of spin-orbit coupling in stabilizing metallic and insulating states and introduces the orbitally-selective non-Fermi liquid phase in correlated multi-band systems.

Findings

SOC enhances orbital hybridization favoring metallicity at low interactions.

SOC stabilizes Mott insulating states at higher interactions.

An orbitally-selective non-Fermi liquid state coexists with other phases.

Abstract

We show that spin-orbit coupling (SOC) plays Janus-faced roles on the orbitally-selective Mott transitions in a three-orbital Hubbard model with crystal field splitting at a specific filling of $2/3$, which is a minimal Hamiltonian for ruthenates. While the SOC favors metallic state due to enhancement of orbital hybridization at smaller on-site Coulomb repulsions, it stabilizes the Mott insulating state ascribed to lifting of orbital degeneracies and enhancement of band polarizations at larger electronic interaction. Moreover, an orbitally-selective non-Fermi liquid (OSnFL), where breakdown and retention of the Fermi liquid coexist in different orbitals, emerges between the orbitally-selective Mott phase and the Fermi-liquid state. This novel state can be used to account for the exotic metallic behavior observed in 4$d$ materials, such as Ca$_{1.8}$Sr$_{0.2}$RuO$_4$, Ba$_2$RuO$_4$ under strain and Sr$_2$RuO$_4$ under uniaxial pressure. We propose that orbitally-selective Kondo breakdown may account for the OSnFL.