A New Era of Excitonic Insulators

TL;DR

This paper reviews recent experimental and theoretical progress in excitonic insulators, highlighting new materials, measurement techniques, and the complex interplay of electron-hole correlations, lattice distortions, and collective modes.

Contribution

It provides a comprehensive overview of recent developments in understanding and identifying excitonic insulators in real materials, bridging theory and experiment.

Findings

Identification of candidate materials with excitonic insulator states

Observation of phase transitions involving lattice distortions

Analysis of collective modes as dynamical signatures

Abstract

The fundamental idea of the excitonic insulator (EI) driven by electron-hole correlations in narrow-gap semiconductors or semimetals was originally proposed in the 1960s, and only theoretical studies had been advanced for a long time. However, the rise of new candidate materials and recent developments in measurement techniques have enabled us to discuss the possibilities of EI states in real materials experimentally. In this article, we review recent progress in the research of EIs. We start with an introduction to the theoretical background of the EI and the mechanism of the order formation including its relation to the physics of the Bardeen-Cooper-Schrieffer (BCS) - Bose-Einstein condensation (BEC) crossover. We also review the EI states studied in the context of strongly correlated electron systems. Then, we introduce the candidate materials for the EI and the issues raised by recent experiments. For example, the phase transitions in several candidate materials are accompanied by lattice distortions, and the contributions from electron-lattice coupling hinder the identification of the excitonic entity. We also review the profiles of the collective modes to discuss the dynamical signatures of the EI.