Domain-like Incommensurate Charge-Density-Wave States and the First-Order Incommensurate-Commensurate Transitions in Layered Tantalum Dichalcogenides. I. 1T-Polytype

The incommensurate (I) states of the triple charge density waves (CDW) in 1T-TaS 2 are studied in details by taking into account a sufficiently large number of harmonics. A domain-like structure of the I-CDW state is shown to be realized as the fundamental wave vector of the I state approaches to the commensurate (“3×1” superstructure) wave vector; the domain boundary is characterized generally by a change of the phase and amplitude of the CDW, and inside the domain an approximately commensurate CDW is stabilized. The nature of the commensurability energy and the coexistence of three CDW's are reflected in the pattern of the domain-like structure. The successive first-order transitions in 1T-TaS 2 are discussed in this connection. The presence of a domain-like structure, which our theory predicts for the nearly commensurate phase of 1T-TaS 2 , is consistent with the recent X-ray photoemission experiment by Hughes and Pollak.