Anomalous excitonic phase diagram in band-gap-tuned Ta2Ni(Se,S)5

Cheng Chen, Weichen Tang, Xiang Chen, Zhibo Kang, Shuhan Ding, Kirsty Scott, Siqi Wang, Zhenglu Li, Jacob P. C. Ruff, Makoto Hashimoto, Dong-Hui Lu, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Eduardo H. Silva Neto, Robert J. Birgeneau, Yulin Chen, Steven G. Louie (), Yao Wang () and Yu He ()
Additional contact information
Cheng Chen: University of Oxford
Weichen Tang: University of California
Xiang Chen: University of California
Zhibo Kang: Yale University
Shuhan Ding: Clemson University
Kirsty Scott: Yale University
Siqi Wang: Yale University
Zhenglu Li: University of California
Jacob P. C. Ruff: Cornell University
Makoto Hashimoto: SLAC National Accelerator Laboratory
Dong-Hui Lu: SLAC National Accelerator Laboratory
Chris Jozwiak: Lawrence Berkeley National Laboratory
Aaron Bostwick: Lawrence Berkeley National Laboratory
Eli Rotenberg: Lawrence Berkeley National Laboratory
Eduardo H. Silva Neto: Yale University
Robert J. Birgeneau: University of California
Yulin Chen: University of Oxford
Steven G. Louie: University of California
Yao Wang: Clemson University
Yu He: Yale University

Nature Communications, 2023, vol. 14, issue 1, 1-7

Abstract: Abstract During a band-gap-tuned semimetal-to-semiconductor transition, Coulomb attraction between electrons and holes can cause spontaneously formed excitons near the zero-band-gap point, or the Lifshitz transition point. This has become an important route to realize bulk excitonic insulators – an insulating ground state distinct from single-particle band insulators. How this route manifests from weak to strong coupling is not clear. In this work, using angle-resolved photoemission spectroscopy (ARPES) and high-resolution synchrotron x-ray diffraction (XRD), we investigate the broken symmetry state across the semimetal-to-semiconductor transition in a leading bulk excitonic insulator candidate system Ta2Ni(Se,S)5. A broken symmetry phase is found to be continuously suppressed from the semimetal side to the semiconductor side, contradicting the anticipated maximal excitonic instability around the Lifshitz transition. Bolstered by first-principles and model calculations, we find strong interband electron-phonon coupling to play a crucial role in the enhanced symmetry breaking on the semimetal side of the phase diagram. Our results not only provide insight into the longstanding debate of the nature of intertwined orders in Ta2NiSe5, but also establish a basis for exploring band-gap-tuned structural and electronic instabilities in strongly coupled systems.

Date: 2023
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DOI: 10.1038/s41467-023-43365-1

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