Exciton Transport in the Electron-hole System Ta₂NiSe₅

A ternary transition metal chalcogenide, Ta₂NiSe₅ has been recently proposed as a candidate of the Excitonic insulator(EI). An extremely flattened band dispersion in a single particle excitation spectrum observed by an angle resolved photoemission spectroscopy is regarded as evidence of the EI phase in this compound[1]. However, macroscopic physical property to evidence the spontaneously generated electron-hole pairs in Ta₂NiSe₅ have not been found yet.
In this time we focus on the dielectric constant and thermopower of single crystals of Ta₂NiSe₅. The dielectric constant characterizes the charge response upon an ac field that rocks an electron-hole pair. In particular, when the electron-hole pairs are tightly bound, they can behave as permanent electric dipoles and should exhibit a peculiar ac response. Similarly a temperature gradient couples to excitons.
The dielectric constant below 50 K shows relaxor-like relaxation, implying the existence of randomly distributed electric dipoles. Furthermore, a large thermopower of 600 μV/K at 100 K suddenly drops toward zero down to 50 K. We ascribe these highly unconventional transport properties in Ta₂NiSe₅ at low temperatures to exciton transport.

[1]Y. Wakisaka, et al., Phys. Rev. Lett. 103, 026402 (2009).