Temperature-dependent excitonic superfluid plasma frequency evolution in an excitonic insulator, Ta2NiSe5

Abstract

Abstract An interesting van der Waals material, Ta2NiSe5 has been known one of strong excitonic insulator candidates since it has very small or zero bandgap and can have a strong exciton binding energy because of its quasi-one-dimensional crystal structure. Here we investigate a single crystal Ta2NiSe5 using optical spectroscopy. Ta2NiSe5 has quasi-one-dimensional chains along the a-axis. We have obtained anisotropic optical properties of a single crystal Ta2NiSe5 along the a- and c-axes. The measured a- and c-axis optical conductivities exhibit large anisotropic electronic and phononic properties. With regard to the a-axis optical conductivity, a sharp peak near 3050 cm−1 at 9 K, with a well-defined optical gap ($${{\boldsymbol{\Delta }}}_{{\boldsymbol{o}}{\boldsymbol{p}}}^{{\boldsymbol{E}}{\boldsymbol{I}}}\,{\boldsymbol{\simeq }}$$ ΔopEI≃ 1800 cm−1) and a strong temperature-dependence, is observed. With an increase in temperature, this peak broadens and the optical energy gap closes around ∼325 K ($${{\boldsymbol{T}}}_{{\boldsymbol{c}}}^{{\boldsymbol{E}}{\boldsymbol{I}}}$$ TcEI ). The spectral weight redistribution with respect to the frequency and temperature indicates that the normalized optical energy gap $$({{\boldsymbol{\Delta }}}_{{\boldsymbol{o}}{\boldsymbol{p}}}^{{\boldsymbol{E}}{\boldsymbol{I}}}({\boldsymbol{T}})/\,{{\boldsymbol{\Delta }}}_{{\boldsymbol{o}}{\boldsymbol{p}}}^{{\boldsymbol{E}}{\boldsymbol{I}}}{\bf{(0)}})$$ (ΔopEI(T)/ΔopEI(0)) is $${\bf{1}}{\boldsymbol{-}}{({\boldsymbol{T}}/{{\boldsymbol{T}}}_{{\boldsymbol{c}}}^{{\boldsymbol{E}}{\boldsymbol{I}}})}^{{\bf{2}}}$$ 1−(T/TcEI)2 . The temperature-dependent superfluid plasma frequency of the excitonic condensation in Ta2NiSe5 has been determined from measured optical data. Our study may pave new avenues in the future research on excitonic insulators.