Electronic Instability in a Zero-Gap Semiconductor: The Charge-Density Wave in (TaSe_4)_2I

摘要

We report a comprehensive study of the paradigmatic quasi-ID compound (TaSe_4)_2I performed by means of angle-resolved photoemission spectroscopy (ARPES) and first-principles electronic structure calculations. We find it to be a zero-gap semiconductor in the nondistorted structure, with non-negligible interchain coupling. Theory and experiment support a Peierls-like scenario for the charge-density wave formation below r_(CDW) = 263 K, where the incommensurability is a direct consequence of the finite interchain coupling. The formation of small polarons, strongly suggested by the ARPES data, explains the puzzling semiconductor-to-semiconductor transition observed in transport at T_(CDW).