Charge density wave states in tantalum dichalcogenides

Using density functional theory, we explore a range of charge density wave states (CDWs) in tantalum-based transition-metal dichalcogenide monolayers. The high-symmetry states of the $1H$ phases of $\mathrm{Ta}{X}_{2}$ ($X$ = S, Se, Te) are lower in total energy compared to the $1T$ variants, while the $1T$ phases exhibit a much stronger tendency for CDW formation. The stability of several CDWs is found to be stronger as the chalcogenide is changed in the sequence (S, Se, Te), with the tellurium-based systems exhibiting several CDWs with binding energy per formula unit in the range of $100\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$. These $1T$ CDW phases are lower in energy than the corresponding $1H$ CDW phases. The diversity of CDWs exhibited by these materials suggests that many ``hidden'' states may occur on ultrafast excitation or photodoping. Changes in electronic structure across the $\mathrm{Ta}{X}_{2}$ series are also elucidated.