A New Era of Excitonic Insulators

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.

23 pages, 16 figures, invited review paper

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• Suppression of the charge fluctuations by nonlocal correlations close to the Mott transition
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• Magnetic-field effect on excitonic condensation emergent in extended Falicov-Kimball model
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• Excitons and Optical Response in Excitonic Insulator Candidate TiSe${}_2$
• Interplay of Rashba spin-orbit coupling and Coulomb interaction in topological spin-triplet excitonic condensates
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• Stabilizing Itinerant Electrons in a Corner-Sharing Kagomé Oxide Nd4Os3ZnO14
• Ab initio Approach to Collective Excitations in Excitonic Insulators