2 x 2 Charge Density Wave in single-layer TiTe2

Abstract

A density functional theory study concerning the origin of the recently reported 2 2 charge density wave (CDW) instability in single-layer TiTe2 is reported. It is shown that, whereas calculations employing the semi-local functional PBE favor the undistorted structure, the hybrid functional HSE06 correctly predicts a 2 2 distortion. The study suggests that the magnitude of the semi-metallic overlap between the valence band top at and the conduction band bottom at M is a key factor controlling the tendency towards the distortion. It is also shown that tensile strain stabilizes a 2 2 CDW, and we suggest that this fact could be further used to induce the instability in double-layers of TiTe2, which in the absence of strain remain undistorted in the experiment. The driving force for the CDW instability seems to be the same phonon mediated mechanism acting for single-layer TiSe2, although in single-layer TiTe2 the driving force is smaller, and the semimetallic character is kept below the transition temperature.