Journal Article

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

Louie, Steven G., Kang, Zhibo, Hashimoto, Makoto, Scott, Kirsty, Wang, Siqi, Ding, Shuhan, Rotenberg, Eli, Birgeneau, Robert J., He, Yu, Ruff, Jacob P. C., Chen, Xiang, Jozwiak, Chris, Chen, Yulin, Li, Zhenglu, da Silva Neto, Eduardo H., Wang, Yao, Chen, Cheng, Lu, Dong-Hui, Tang, Weichen, Bostwick, Aaron

2023

Overview

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 Ta 2 Ni(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 Ta 2 NiSe 5 , but also establish a basis for exploring band-gap-tuned structural and electronic instabilities in strongly coupled systems. The presence of excitonic instability and its relationship with a structural transition in Ta 2 NiSe 5 has been debated. Chen et al. map out the electronic bands and lattice distortion across the semimetal-to-semiconductor transition with sulfur doping, revealing the crucial role of electron-phonon coupling.

• Publisher: Nature Publishing Group UK,Nature Publishing Group,Nature Portfolio
• Subject: 639/301/119/995 / 639/766/119/2795 / Broken symmetry / Coupling / electronic properties and materials / Electrons / Energy gap / Excitons / First principles / Humanities and Social Sciences / Insulators / MATERIALS SCIENCE / multidisciplinary / Phase diagrams / phase transitions and critical phenomena / Phonons / Photoelectric emission / Science / Science (multidisciplinary) / Selenium / Spectroscopy / Sulfur / Symmetry / Synchrotron radiation / Synchrotrons / Transition points / X-ray diffraction