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

TL;DR

This study investigates the excitonic phase transition in Ta2Ni(Se,S)5 using ARPES and XRD, revealing a continuous suppression of broken symmetry and highlighting the role of electron-phonon coupling in a bulk excitonic insulator.

Contribution

It provides new insights into the excitonic phase diagram and the influence of electron-phonon interactions across the semimetal-to-semiconductor transition in Ta2NiSe5.

Findings

Broken symmetry phase is continuously suppressed across the transition.

Strong interband electron-phonon coupling influences symmetry breaking.

Results challenge the expectation of maximal excitonic instability at the Lifshitz point.

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.