Pressure dependence of ultrafast carrier dynamics in excitonic insulator Ta2NiSe5

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 Ta2NiSe5 up 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.