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

TL;DR

This study investigates how applying pressure affects the electronic properties of Ta2NiSe5, revealing a transition from an excitonic insulator to a semimetallic state through optical conductivity measurements up to 10 GPa.

Contribution

It provides the first detailed pressure-dependent optical conductivity data for Ta2NiSe5, elucidating the suppression of the excitonic insulator state under pressure.

Findings

Energy gap narrows with pressure

Excitonic peak broadens and diminishes

Material transitions to a semimetal above 3 GPa

Abstract

The layered chalcogenide Ta2NiSe5 has recently attracted much interest as a strong candidate for the long sought excitonic insulator (EI). Since the physical properties of an EI are expected to depend sensitively on the external pressure, it is important to clarify the pressure evolution of microscopic electronic state in Ta2NiSe5. Here we report the optical conductivity [s(w)] of Ta2NiSe5 measured at high pressures to 10 GPa and at low temperatures to 8 K. With cooling at ambient pressure, s(w) develops an energy gap of about 0.17 eV and a pronounced excitonic peak at 0.38 eV, as already reported in the literature. Upon increasing pressure, the energy gap becomes narrower and the excitonic peak is broadened. 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 pressure. At higher pressures, s(w) exhibits metallic characteristics with no energy gap. The detailed pressure evolution of s(w) is presented, and discussed mainly in terms of a weakening of excitonic correlation with pressure.