Theory of the thickness dependence of the charge density wave transition in 1 T-TiTe2

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 1 T-TiTe(2)is a remarkable exception as it shows no evidence of charge density wave formation in bulk, but it displays a stable 2 x 2 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 possibleformation mechanisms. Here, by performingnon-perturbative anharmonic calculations with gradient corrected and hybrid functionals, we explain the thickness behaviour of the transition in 1 T-TiTe. We demonstrate that 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 active phonon modes.