Semimetal-to-semimetal charge density wave transition in 1T-TiSe2

We report an infrared study on 1T-TiSe2, the parent compound of the newly discovered superconductor CuxTiSe2. Previous studies of this compound have not conclusively resolved whether it is a semimetal or a semiconductor - information that is important in determining the origin of its unconventional charge density wave (CDW) transition. Here we present optical spectroscopy results that clearly reveal that the compound is metallic in both the high-temperature normal phase and the low-temperature CDW phase. The carrier scattering rate is dramatically different in the normal and CDW phases and the carrier density is found to change with temperature. We conclude that the observed properties can be explained within the scenario of an Overhauser-type CDW mechanism.