Direct observation of excitonic instability in Ta2NiSe5

摘要: Coulomb attraction between electrons and holes in a narrow-gap semiconductor or semimetal is predicted to lead an elusive phase of matter dubbed `excitonic insulator'. However, direct observation such electronic instability remains extremely rare. Here, we report the incipient divergence static excitonic susceptibility candidate material Ta2NiSe5 using Raman spectroscopy. Critical fluctuations order parameter give rise quasi-elastic scattering B2g symmetry, whose intensity grows inversely with temperature toward Weiss Tw ~237 K, which arrested by structural transition driven acoustic phonon same symmetry at Tc =325 K. Concurrently, optical becomes heavily damped extent that its trace almost invisible around TC, manifests strong electron-phonon coupling has obscured identification low-temperature as insulator for more than decade. Our result unambiguously reveals origin transition.