Role of electron-phonon coupling in excitonic insulator candidate ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$

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Role of electron-phonon coupling in excitonic insulator candidate ${Ta}_{2} {NiSe}_{5}$

Cheng Chen, Xiang Chen, Weichen Tang, Zhenglu Li, Siqi Wang, Shuhan Ding, Zhibo Kang, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Makoto Hashimoto, Donghui Lu, Jacob P. C. Ruff, Steven G. Louie, Robert J. Birgeneau, Yulin Chen, Yao Wang, and Yu He

Phys. Rev. Research 5, 043089 – Published 26 October 2023

Abstract

Electron-hole bound pairs, or excitons, are common excitations in semiconductors. They can spontaneously form and condense into a new insulating ground state—the so-called excitonic insulator—when the energy of electron-hole Coulomb attraction exceeds the band gap. In the presence of electron-phonon coupling, a periodic lattice distortion often concomitantly occurs. However, a similar structural transition can also be induced by electron-phonon coupling itself, therefore hindering the clean identification of bulk excitonic insulators (e.g., which instability is the driving force of the phase transition). Using high-resolution synchrotron x-ray diffraction and angle-resolved photoemission spectroscopy, we identify key electron-phonon coupling effects in a leading excitonic insulator candidate ${Ta}_{2} {NiSe}_{5}$ . These include an extensive unidirectional lattice fluctuation and an electronic pseudogap in the normal state, as well as a negative electronic compressibility in the charge-doped broken-symmetry state. In combination with first principles and model calculations, we use the normal state electronic spectra to quantitatively determine the electron-phonon interaction vertex $g$ and interband Coulomb interaction $V$ in the minimal lattice model, the solution to which captures the experimental observations. Moreover, we show how the Coulomb and electron-phonon coupling effects can be unambiguously separated based on the solution to quantified microscopic models. Finally, we discuss how the strong lattice fluctuations enabled by low dimensionality relate to the unique electron-phonon interaction effects beyond the textbook Born-Oppenheimer approximation..

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Received 9 April 2023
Accepted 11 October 2023

DOI:https://doi.org/10.1103/PhysRevResearch.5.043089

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Electron-phonon coupling Electronic structure Excitons Metal-insulator transition Phase transitions

Angle-resolved photoemission spectroscopy Exact diagonalization First-principles calculations Resistivity measurements X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Cheng Chen^1,2,*, Xiang Chen ^3,4,*, Weichen Tang ^3,4, Zhenglu Li^3,4, Siqi Wang², Shuhan Ding ⁵, Zhibo Kang ², Chris Jozwiak ⁶, Aaron Bostwick⁶, Eli Rotenberg⁶, Makoto Hashimoto⁷, Donghui Lu⁷, Jacob P. C. Ruff⁸, Steven G. Louie ^3,4, Robert J. Birgeneau^3,4,9, Yulin Chen¹, Yao Wang^5,10,†, and Yu He ^2,‡

¹Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom
²Department of Applied Physics, Yale University, New Haven, Connecticut 06511, USA
³Physics Department, University of California, Berkeley, California 94720, USA
⁴Materials Sciences Division, Lawrence Berkeley National Lab, Berkeley, California 94720, USA
⁵Department of Physics and Astronomy, Clemson University, Clemson, South Carolina 29631, USA
⁶Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁷Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
⁸Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York 14853, USA
⁹Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA
¹⁰Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA

^*These authors contributed equally to this work.
^†yao.wang@emory.edu
^‡yu.he@yale.edu

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Vol. 5, Iss. 4 — October - December 2023

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Role of electron-phonon coupling in excitonic insulator candidate Ta2NiSe5

Cheng Chen, Xiang Chen, Weichen Tang, Zhenglu Li, Siqi Wang, Shuhan Ding, Zhibo Kang, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Makoto Hashimoto, Donghui Lu, Jacob P. C. Ruff, Steven G. Louie, Robert J. Birgeneau, Yulin Chen, Yao Wang, and Yu He

Phys. Rev. Research 5, 043089 – Published 26 October 2023

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Role of electron-phonon coupling in excitonic insulator candidate ${Ta}_{2} {NiSe}_{5}$