Dimensionality-driven phonon softening and incipient charge density wave instability in TiS<inf>2</inf>

Abstract

Density functional theory and density functional perturbation theory are used to investigate the electronic and vibrational properties of TiS 2 . Within the local density approxima- tion the material is a semimetal both in the bulk and in the monolayer form. Most interestingly we observe a Kohn anomaly in the bulk phonon dispersion, which turns into a charge density wave instability when TiS 2 is thinned to less than four monolayers. Such instability, however, disappears when one calculates the electronic structure with a functional, such as the LDA+ U , which returns an insulating ground state. In this situation charge-doping or strain does not bring back the charge density wave instability, whereas the formation of the TiSSe alloy does.