Dynamical behavior across the Mott transition of two bands with different bandwidths

TL;DR

This study explores how differences in bandwidth affect the Mott transition in a two-band Hubbard model, revealing complex spectral behaviors and the potential coexistence of metallic and insulating states.

Contribution

It compares Gutzwiller variational and dynamical mean-field theory approaches to understand the Mott transition in systems with bandwidth disparity.

Findings

Narrower band undergoes Mott transition before wider band in variational calculations.

Spectral weight persists inside the gap even after the quasiparticle residue vanishes.

A narrow quasiparticle resonance at the chemical potential likely coexists with the insulating state.

Abstract

We investigate the role of the bandwidth difference in the Mott metal-insulator transition of a two-band Hubbard model in the limit of infinite dimensions, by means of a Gutzwiller variational wave function as well as by dynamical mean-field theory. The variational calculation predicts a two-stage quenching of the charge degrees of freedom, in which the narrower band undergoes a Mott transition before the wider one, both in the presence and in the absence of a Hund's exchange coupling. However, this scenario is not fully confirmed by the dynamical mean-field theory calculation, which shows that, although the quasiparticle residue of the narrower band is zero within our numerical accuracy, low-energy spectral weight still exists inside the Mott-Hubbard gap, concentrated into two peaks symmetric around the chemical potential. This spectral weight vanishes only when the wider band ceases to conduct too. Although our results are compatible with several scenarios, e.g., a narrow gap semiconductor or a semimetal, we argue that the most plausible one is that the two peaks coexist with a narrow resonance tied at the chemical potential, with a spectral weight below our numerical accuracy. This quasiparticle resonance is expected to vanish when the wider band undergoes the Mott transition.