Dimensionality-driven phonon softening and incipient charge density wave instability in TiS₂

School of Physics, AMBER and CRANN Institute, Trinity College - Dublin 2, Ireland

^(a) doluik@tcd.ie (corresponding author)

Received: 25 January 2016
Accepted: 25 August 2016

Abstract

Density functional theory and density functional perturbation theory are used to investigate the electronic and vibrational properties of TiS₂. Within the local density approximation 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 TiS₂ 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.