Theory of the thickness dependence of the charge density wave transition in 1T-TiTe$_2$

TL;DR

This paper explains why single-layer 1T-TiTe2 exhibits a charge density wave transition while bulk does not, by analyzing anharmonic effects and exchange interactions that enhance electron-phonon coupling in the monolayer.

Contribution

It provides a non-perturbative theoretical explanation for the thickness-dependent CDW transition in 1T-TiTe2, highlighting the role of anharmonicity and exchange interactions.

Findings

CDW occurs in single-layer TiTe2 due to anharmonicity and exchange-enhanced electron-phonon interaction.

The study characterizes the electronic structure and phonon modes of the single-layer CDW phase.

It clarifies the mechanism behind the 3D-2D crossover of the CDW transition in TiTe2.

Abstract

Most metallic transition metal dichalcogenides undergo charge density wave (CDW) instabilities with similar or identical ordering vectors in bulk and in single layer, albeit with different critical temperatures. Metallic 1T-TiTe$_2$ is a remarkable exception as it shows no evidence of charge density wave formation in bulk, but it displays a stable $2\times2$ reconstruction in single-layer form. The mechanism for this 3D-2D crossover of the transition is still unclear, although strain from the substrate and the exchange interaction have been pointed out as possible formation mechanisms. Here, by performing non-perturbative anharmonic calculations with gradient corrected and hybrid functionals, we explain the thickness behaviour of the transition in 1T-TiTe$_2$. We demonstrate that the occurrence of the CDW in single-layer TiTe$_2$ occurs from the interplay of non-perturbative anharmonicity and an exchange enhancement of the electron-phonon interaction, larger in the single layer than in the bulk. Finally, we study the electronic and structural properties of the single-layer CDW phase and provide a complete description of its electronic structure, phonon dispersion as well as infrared and Raman active phonon modes.