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

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 ([Formula: see text] 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 ([Formula: see text]). The spectral weight redistribution with respect to the frequency and temperature indicates that the normalized optical energy gap ([Formula: see text]) is [Formula: see text]. 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.