Temperature-dependent evolutions of excitonic superfluid plasma frequency in a srong excitonic insulator candidate, Ta${}_2$NiSe${}_5$

Abstract

We investigate an interesting anisotropic van der Waals material, Ta${}_2$NiSe${}_5$, using optical spectroscopy. Ta${}_2$NiSe${}_5$ has been known as one of the few excitonic insulators proposed over 50 years ago. Ta${}_2$NiSe${}_5$ has quasi-one dimensional chains along the $a$-axis. We have obtained anisotropic optical properties of a single crystal Ta${}_2$NiSe${}_5$ 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 (${\Delta }^{EI}\simeq$ 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 $\sim$325 K($T_c^{EI}$). The spectral weight redistribution with respect to the frequency and temperature indicates that the normalized optical energy gap (${\Delta }^{EI}(T)/{\Delta }^{EI}(0)$) is $1-(T/T_c^{EI}{)}^2$. The temperature-dependent superfluid plasma frequency of the excitonic condensation in Ta${}_2$NiSe${}_5$ has been determined from measured optical data. Our findings may be useful for future research on excitonic insulators.

17 pages, 5 figures