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

Abstract

The layered chalcogenide ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ 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 ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$. Here we report the optical conductivity $[\ensuremath{\sigma}(\ensuremath{\omega})$] of ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ measured at high $P$ to 10 GPa and at low temperatures to 8 K. With cooling at $P=0$, $\ensuremath{\sigma}(\ensuremath{\omega})$ 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 ${P}_{s}\ensuremath{\simeq}3$ GPa, the energy gap becomes partially filled, indicating that ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ is a semimetal after the EI state is suppressed by $P$. At higher $P$, $\ensuremath{\sigma}(\ensuremath{\omega})$ exhibits metallic characteristics with no energy gap. The detailed $P$ evolution of the energy gap and $\ensuremath{\sigma}(\ensuremath{\omega})$ is presented, and discussed mainly in terms of a weakening of excitonic correlation with $P$.