Collective modes in excitonic insulators: Effects of electron-phonon coupling and signatures in the optical response
Abstract
Here, we consider a two-band spinless model describing an excitonic insulator (EI) on the two-dimensional square lattice with anisotropic hopping parameters and electron-phonon (el-ph) coupling, inspired by the EI candidate Ta2NiSe5. We systematically study the nature of the collective excitations in the ordered phase which originates from the interband Coulomb interaction and the el-ph coupling. When the ordered phase is stabilized only by the Coulomb interaction (pure EI phase), its collective response exhibits a massless phase mode in addition to the amplitude mode. We show that in the BEC regime, the signal of the amplitude mode becomes less prominent and that the anisotropy in the phase-mode velocities is reduced. Through coupling to the lattice, the phase mode acquires a mass and the signal of the amplitude mode becomes less prominent. Importantly, the character of the softening mode at the boundary between the normal semiconductor phase and the ordered phase depends on the parameters. In particular, we point out that even for el-ph coupling smaller than the Coulomb interaction, the mode that softens to zero at the boundary can have a phonon character. We also discuss how the collective modes can be observed in the optical conductivity. Furthermore, we study themore »
- Authors:
-
[2];
- Tokyo Institute of Technology (Japan)
- Flatiron Institute, New York, NY (United States)
- Columbia Univ., New York, NY (United States)
- Flatiron Institute, New York, NY (United States); Columbia Univ., New York, NY (United States)
- University of Fribourg (Switzerland)
- Publication Date:
- Research Org.:
- Columbia Univ., New York, NY (United States); Energy Frontier Research Centers (EFRC) (United States). Programmable Quantum Materials (Pro-QM)
- Sponsoring Org.:
- USDOE Office of Science (SC), Basic Energy Sciences (BES); Japan Society for the Promotion of Science (JSPS); Japan Science and Technology Agency (JST); European Research Council (ERC); Swiss National Science Foundation (SNF)
- OSTI Identifier:
- 1803827
- Grant/Contract Number:
- SC0019443; JP19K23425; JP18K13509; JP19H05821; JP18K04678; JP17K05536; JPMJCR1901; 724103
- Resource Type:
- Accepted Manuscript
- Journal Name:
- Physical Review. B
- Additional Journal Information:
- Journal Volume: 101; Journal Issue: 19; Journal ID: ISSN 2469-9950
- Publisher:
- American Physical Society (APS)
- Country of Publication:
- United States
- Language:
- English
- Subject:
- 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; Materials Science; Physics
Citation Formats
Murakami, Yuta, Golež, Denis, Kaneko, Tatsuya, Koga, Akihisa, Millis, Andrew J., and Werner, Philipp. Collective modes in excitonic insulators: Effects of electron-phonon coupling and signatures in the optical response. United States: N. p., 2020.
Web. doi:10.1103/physrevb.101.195118.
Murakami, Yuta, Golež, Denis, Kaneko, Tatsuya, Koga, Akihisa, Millis, Andrew J., & Werner, Philipp. Collective modes in excitonic insulators: Effects of electron-phonon coupling and signatures in the optical response. United States. https://doi.org/10.1103/physrevb.101.195118
Murakami, Yuta, Golež, Denis, Kaneko, Tatsuya, Koga, Akihisa, Millis, Andrew J., and Werner, Philipp. Mon .
"Collective modes in excitonic insulators: Effects of electron-phonon coupling and signatures in the optical response". United States. https://doi.org/10.1103/physrevb.101.195118. https://www.osti.gov/servlets/purl/1803827.
@article{osti_1803827,
title = {Collective modes in excitonic insulators: Effects of electron-phonon coupling and signatures in the optical response},
author = {Murakami, Yuta and Golež, Denis and Kaneko, Tatsuya and Koga, Akihisa and Millis, Andrew J. and Werner, Philipp},
abstractNote = {Here, we consider a two-band spinless model describing an excitonic insulator (EI) on the two-dimensional square lattice with anisotropic hopping parameters and electron-phonon (el-ph) coupling, inspired by the EI candidate Ta2NiSe5. We systematically study the nature of the collective excitations in the ordered phase which originates from the interband Coulomb interaction and the el-ph coupling. When the ordered phase is stabilized only by the Coulomb interaction (pure EI phase), its collective response exhibits a massless phase mode in addition to the amplitude mode. We show that in the BEC regime, the signal of the amplitude mode becomes less prominent and that the anisotropy in the phase-mode velocities is reduced. Through coupling to the lattice, the phase mode acquires a mass and the signal of the amplitude mode becomes less prominent. Importantly, the character of the softening mode at the boundary between the normal semiconductor phase and the ordered phase depends on the parameters. In particular, we point out that even for el-ph coupling smaller than the Coulomb interaction, the mode that softens to zero at the boundary can have a phonon character. We also discuss how the collective modes can be observed in the optical conductivity. Furthermore, we study the effects of nonlocal interactions on the collective modes and show the possibility of realizing a coexistence of an in-gap mode and an above-gap mode split off from the single-amplitude mode in the system with local interactions only.},
doi = {10.1103/physrevb.101.195118},
journal = {Physical Review. B},
number = 19,
volume = 101,
place = {United States},
year = {Mon May 11 00:00:00 EDT 2020},
month = {Mon May 11 00:00:00 EDT 2020}
}
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Imaging the coherent propagation of collective modes in the excitonic insulator candidate Ta$_2$NiSe$_5$ at room temperature
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