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 TaX2 (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 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 TaX2 series are also elucidated.