Excitonic insulator is an elusive phase of matter predicted many decades ago to occur in a narrow gap semiconductor or a semi-metal. Analogous to Cooper pairs in superconductors, Coulomb attractions bind electrons and holes in pairs to form charge-neutral excitons, which undergo a Bose-Einstein condensation at a sufficiently low temperature. However, unambiguous identification of an excitonic insulator remains challenging because candidate materials invariably display simultaneous structural phase transitions. In this talk, I will discuss the case of Ta2NiSe5, for which a fierce debate continues for more than a decade on the physical origin of its semimetal-to-insulator transition. Using Raman scattering, we have observed an incipient divergence in the uniform static electronic susceptibility. Critical fluctuations of the excitonic order give rise to quasi-elastic scattering of B2g symmetry, whose intensity grows inversely with temperature toward the Weiss temperature of Tw~237 K, which is arrested by a structural phase transition driven by an acoustic phonon of the same symmetry at Tc=325 K. Concurrently, a B2g optical phonon becomes heavily overdamped to the extent that its trace is almost invisible around TC, which manifests a strong electron- phonon coupling that has obscured the identification of the low-temperature phase as an excitonic insulator for more than a decade. Our result unambiguously reveals the electronic origin of the phase transition.

Chair: Yuki Motome