Pressure dependence of ultrafast carrier dynamics in excitonic insulator Ta₂NiSe₅

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 Ta₂NiSe₅up to 5 GPa. The differential reflectivity as a function of delay time between the pump and probe pulses shows two relaxation processes with their time constants and amplitudes revealing changes at PC1∼1 GPa (transition from EI phase to semiconductor) and PC2∼3 GPa (from semiconductor to semimetallic phase). The pressure dependence of the fast relaxation time and corresponding amplitude in the EI phase are captured by the Rothwarf-Taylor model, bringing out the decrease of the bandgap under pressure, with a pressure coefficient of 65 meV GPa^-1, closely agreeing with our first principle calculations. The decrease of the slow relaxation time in the EI phase with pressure is due to enhanced electron-phonon coupling as confirmed by our calculations. The fluence dependence of the relaxation parameters at different pressures corroborates the semi-metallic nature above PC2. Our experiments combined with first principle calculations thus provide additional insights into different high-pressure phases of Ta₂NiSe₅.