Excitonic insulator and its failed quantum phase transition in Ta2Ni(Se1-xSx)5

Excitonic insulator (EI) is a phase driven by Coulomb attraction between electrons and holes leading to a proliferation of particle-hole pairs. However, excitonic insulators break lattice symmetries, raising the question of whether a particular transition is excitonic or structural. I will show how electronic Raman scattering can be used to elucidate the transition origin in the Ta2Ni(Se1-xSx)5 family of candidate materials. In particular, I will demonstrate an excitonic-driven transition at low x transforming into a structural transition at x=1. The study reveals a quantum phase transition of an excitonic insulator masked by a preemptive structural order.

arXiv:2104.07032; PRB 104, 045102 (2021); npj Quantum Materials 6:52 (2021)