Observation of Charge Density Wave in Layered Hexagonal Cu_1.89Te Single Crystal

We report comprehensive transport, electron microscopy and Raman spectroscopy studies on transition-metal chalcogenides Cu_1.89Te single crystals. The metallic Cu_1.89Te displays successive metal-semiconductor transitions at low temperatures and almost ideal linear MR when magnetic field up to 33 T. Through the electron diffraction patterns, the stable room-temperature phase is identified as a 3 \times 3\times 2 modulated superstructure based on the Nowotny hexagonal structure. The superlattice spots of transmission electron microscopy and scanning tunneling microscopy clearly show the structural transitions from the room-temperature commensurate I phase, named as C-I phase, to the low temperature commensurate II (C-II) phase. All the results can be understood in terms of charge density wave (CDW) instability, yielding intuitive evidences for the CDW formations in Cu_1.89Te. The additional Raman modes below room temperature further reveal that the zone-folded phonon modes may play an important role on the CDW transitions. Our research sheds light on the novel electron features of Cu_1.89Te at low temperature, and may provide potential applications for future nano-devices.