Pressure Dependence of Ultrafast Carrier Dynamics in Excitonic Insulator Ta${}_2$NiSe${}_5$

Abstract

An excitonic insulator (EI) phase is a consequence of collective many-body effects where an optical band gap is formed by the condensation of electron-hole pairs or excitons. We report pressure-dependent optical pump optical probe spectroscopy of EI Ta${}_2$NiSe${}_5$ in an on-site in situ geometry. The fast relaxation process depicts the transition across P${}_{C_1}$ $\sim$1 GPa from EI phase to a semiconductor and P${}_{C_2}$ $\sim$3 GPa from a semiconductor to a semimetallic phase. The instability of the EI phase beyond P${}_{C_1}$ is captured by the Rothwarf-Taylor model by incorporating the decrease of the bandgap under pressure. The pressure coefficient of the bandgap decreases, 65 meV/GPa closely agrees with the first principle calculations.

17 pages, 5 figures