Pressure suppression of the excitonic insulator state in Ta2NiSe5 observed by optical conductivity

The layered chalcogenide Ta2NiSe5 has recently attracted much interest as a strong candidate for a long-sought excitonic insulator (EI). Since the physical properties of an EI are expected to depend sensitively on the external pressure (P), it is important to clarify the P evolution of a microscopic electronic state in Ta2NiSe5. Here we report the optical conductivity [sigma (omega)] of Ta2NiSe5 measured at high P to 10 GPa and at low temperatures to 8 K. With cooling at P = 0, sigma (omega) develops an energy gap of about 0.17 eV and a pronounced excitonic peak at 0.38 eV as reported previously. With increasing P, the energy gap becomes narrower and the excitonic peak is diminished. Above a structural transition at Ps ' 3 GPa, the energy gap becomes partially filled, indicating that Ta2NiSe5 is a semimetal after the EI state is suppressed by P. At higher P, sigma (omega) exhibits metallic characteristics with no energy gap. The detailed P evolution of the energy gap and sigma (omega) is presented, and discussed mainly in terms of a weakening of excitonic correlation with P.

Automated summary

The effect of pressure on the electronic state of Ta2NiSe5 has been studied. It was found that the energy gap becomes narrower and the excitonic peak diminishes with increasing pressure. After a structural transition, Ta2NiSe5 becomes a semimetal. This phenomenon is mainly attributed to the weakening of excitonic correlation with pressure.