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During a band-gap-tuned semimetal-to-semiconductor transition, Coulombattraction between electrons and holes can cause spontaneously formed excitonsnear the zero-band-gap point, or the Lifshitz transition point. This has becomean important route to realize bulk excitonic insulators -- an insulating groundstate distinct from single-particle band insulators. How this route manifestsfrom weak to strong coupling is not clear. In this work, using angle-resolvedphotoemission spectroscopy (ARPES) and high-resolution synchrotron x-raydiffraction (XRD), we investigate the broken symmetry state across thesemimetal-to-semiconductor transition in a leading bulk excitonic insulatorcandidate system Ta2Ni(Se,S)5. A broken symmetry phase is found to becontinuously suppressed from the semimetal side to the semiconductor side,contradicting the anticipated maximal excitonic instability around the Lifshitztransition. Bolstered by first-principles and model calculations, we findstrong interband electron-phonon coupling to play a crucial role in theenhanced symmetry breaking on the semimetal side of the phase diagram. Ourresults not only provide insight into the longstanding debate of the nature ofintertwined orders in Ta2NiSe5, but also establish a basis for exploringband-gap-tuned structural and electronic instabilities in strongly coupledsystems.

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