Orbital selective Mott transition in multi-band systems: slave-spin representation and dynamical mean-field theory

TL;DR

This paper investigates the orbital-selective Mott transition in a two-orbital Hubbard model, revealing conditions under which one orbital localizes before the other and analyzing the stability of the intermediate phase.

Contribution

It introduces a slave-spin mean-field approach and compares it with dynamical mean-field theory to study orbital-selective Mott transitions in multi-band systems.

Findings

Orbital-selective Mott transition occurs for small bandwidth ratios at all Hund couplings.

Intermediate phase at J=0 differs from a conventional Mott insulator with low-energy spectral weight.

Orbital-selective phase is unstable at zero temperature due to inter-orbital hybridization.

Abstract

We examine whether the Mott transition of a half-filled, two-orbital Hubbard model with unequal bandwidths occurs simultaneously for both bands or whether it is a two-stage process in which the orbital with narrower bandwith localizes first (giving rise to an intermediate `orbital-selective' Mott phase). This question is addressed using both dynamical mean-field theory, and a representation of fermion operators in terms of slave quantum spins, followed by a mean-field approximation (similar in spirit to a Gutzwiller approximation). In the latter approach, the Mott transition is found to be orbital-selective for all values of the Coulomb exchange (Hund) coupling J when the bandwidth ratio is small, and only beyond a critical value of J when the bandwidth ratio is larger. Dynamical mean-field theory partially confirms these findings, but the intermediate phase at J=0 is found to differ from a conventional Mott insulator, with spectral weight extending down to arbitrary low energy. Finally, the orbital-selective Mott phase is found, at zero-temperature, to be unstable with respect to an inter-orbital hybridization, and replaced by a state with a large effective mass (and a low quasiparticle coherence scale) for the narrower band.