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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• Finite-temperature properties of excitonic condensation in the extended Falicov-Kimball model: Cluster mean-field-theory approach
• Exciton condensation in bilayer spin-orbit insulator
• Assignment of excitonic insulators in \textit{ab initio} theories: the case of NiBr${}_2$
• Excitonic insulator phase and dynamics of condensate in a topological one-dimensional model
• Excitonic Order and Superconductivity in the Two-Orbital Hubbard Model: Variational Cluster Approach
• Topological $p_x+i{p}_y$ inter-valley coherent state in Moiré MoTe${}_2$/WSe${}_2$ heterobilayers
• Jumps in entropy and magnetic susceptibility at the valence and spin-state transition in a cobalt oxide
• Spin texture and spin current in excitonic phases of the two-band Hubbard model
• A review on magnetic field induced spin crossover in LaCoO${}_3$ up to 600 T
• FFLO Superconductivity Mediated by Excitonic Fluctuation in Semimetallic Ta${}_2$NiSe${}_5$
• Excitonic Correlations, Spin-State Ordering, and Magnetic-Field Effects in One-Dimensional Two-Orbital Hubbard Model for Spin-Crossover Region
• Strong enhancement of magnetic susceptibility induced by spin-nematic fluctuations in an excitonic insulating system with spin-orbit coupling
• Spin loop-current textures in Hubbard models
• Emergence of pure spin current in doped excitonic magnets
• Effects of magnetic fields and orbital angular momentum on excitonic condensation in two-orbital Hubbard model
• Collective mode excitations and simulated $L$-edge resonant-inelastic x-ray scattering spectra in antiferromagnetic Ca${}_2$RuO${}_4$

Cited by in corpus (11)

• First-principles design of excitonic insulators: A review
• Strong long-wavelength electron-phonon coupling in Ta${}_2$Ni(Se,S)${}_5$
• Suppression of the charge fluctuations by nonlocal correlations close to the Mott transition
• Identifying topological excitonic insulators via bulk-edge correspondence
• Magnetic-field effect on excitonic condensation emergent in extended Falicov-Kimball model
• Excitonic Insulator and the Extended Falicov--Kimball Model Away from Half-Filling
• 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
• Raman spectroscopy of van der Waals topological magnet GdGaI
• Stabilizing Itinerant Electrons in a Corner-Sharing Kagomé Oxide Nd4Os3ZnO14
• Ab initio Approach to Collective Excitations in Excitonic Insulators