Correlations in a band insulator

TL;DR

This paper investigates how electron correlations affect a covalent band insulator, revealing a transition to a Mott insulator and showing that correlations can reduce the energy gap, with implications for understanding insulating states.

Contribution

It demonstrates a discontinuous transition from a band insulator to a Mott insulator using dynamical mean-field theory, highlighting the role of correlations in gap renormalization.

Findings

Correlation decreases the energy gap in the band insulator.

Transition from band insulator to Mott insulator is discontinuous with hysteresis.

Spin gap is smaller than charge gap in the correlated phase.

Abstract

We study a model of a covalent band insulator with on-site Coulomb repulsion at half-filling using dynamical mean-field theory. Upon increasing the interaction strength the system undergoes a discontinuous transition from a correlated band insulator to a Mott insulator with hysteretic behavior at low temperatures. Increasing the temperature in the band insulator close to the insulator-insulator transition we find a crossover to a Mott insulator at elevated temperatures. Remarkably, correlations decrease the energy gap in the correlated band insulator. The gap renormalization can be traced to the low-frequency behavior of the self-energy, analogously to the quasiparticle renormalization in a Fermi liquid. While the uncorrelated band insulator is characterized by a single gap for both charge and spin excitations, the spin gap is smaller than the charge gap in the correlated system.