Abstract:

Single-layer transition metal dichalcogenides (TMDCs) can adopt two distinct structures corresponding to different coordination of the metal atoms. TMDCs adopting the T-type structure exhibit a rich and diverse set of phenomena, including charge density waves (CDWs) in a √13 × √13 supercell pattern in TaS₂ and TaSe₂, and a possible excitonic insulating phase in TiSe₂ . These properties make the T-TMDCs desirable components of layered heterostructure devices. In order to predict the emergent properties of combinations of different layered materials, one needs simple and accurate models for the constituent layers which can take into account potential effects of lattice mismatch, relaxation, strain, and structural distortion. Previous studies have developed ab initio tight-binding Hamiltonians for H-type TMDCs [S. Fang et al., Phys. Rev. B 98, 075106
(2018)]. Here we extend this work to include T-type TMDCs. We demonstrate the capabilities and limitations of our model using three example systems: a one-dimensional sinusoidal ripple, which represents a longitudinal acoustic phonon; the 2 × 2 CDW in TiSe₂; and the √13 × √13 CDW in TaS₂. Using the technique of band unfolding we compare the electronic structure of the distorted crystals to the pristine band structure and find our tight-binding model reproduces many features revealed by direct density functional theory calculations, provided the magnitude of the distortions remains in the linear regime. This model of the strain response of single layers is a necessary ingredient for the construction of models of van der Waals heterostructures with multiple layers, because the deformation and strain from mechanical relaxations in a twisted bilayer have important effects on the
electronic structure.

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