Destabilizing excitonic insulator phase by pressure tuning of exciton-phonon coupling

Abstract

Ta 2 NiSe 5 is an excitonic insulator (EI) exhibiting bound electron-hole pairs condensed at room temperature, which transforms to a small-gap semiconducting state above T= 325 K simultaneously undergoing a phonon-related structural transition. Despite the clear experimental evidence for strong exciton-phonon coupling, its role and the origin of EI state in terms of BCS versus Bose-Einstein condensation mechanisms are unclear. Motivated by the tunability of these mechanisms with pressure, we report Raman experiments under pressure of Ta2 NiSe 5 and first-principles theoretical analysis of two pressure-induced transitions at 1 and 3 GPa. We present a simple method to derive the exciton-phonon coupling within density functional theory and show using a model Hamiltonian that reducing strength of this coupling relative to electronic gap and phonon frequency destabilizes the EI state with pressure. In addition to connecting with the Raman anomalies observed under pressure, our simple picture explains the recently observed phonon-coupled state of exciton condensate.