Systems*

Transition metal dichalcogenides (MX₂) exhibit various CDW phases depending on temperature and crystal symmetry. The appearance of stripe CDW phases with anisotropic domain walls [1] has been explained by inter-layer interaction. However, stripe domain walls were observed experimentally in monolayer 1T-TaS₂ [2]. Therefore, a new model for the appearance of anisotropic phases is necessary.

In this talk, we reinvestigate the Ginzburg-Landau model of 2D CDW transition [3]. We show that interference of macroscopic CDW wave functions leads to a universal multivalley free energy structure where various local minima correspond to different CDW phases, including the stripe phases in 1T-TaS₂ and 2H-TaSe₂ [4]. This result indicates the appearance of stripe CDW phases without inter-layer interaction (hence also in monolayer). Then, we use this method to explain a new nearly-commensurate CDW phase in 1H-TaSe₂.

[1] R. M. Fleming et. al., Phys. Rev. Lett. 45, 576 (1980); S. Tanda and T. Sambongi, Synth. Met. 11, 85 (1985)
[2] D. Sakabe, et. al., npj Quant. Mat. 2, 22 (2017)
[3] K. Nakanishi and H. Shiba, J. Phys. Soc. Jpn. 53, 1103 (1984)
[4] K. Nakatsugawa, S. Tanda, and T. N. Ikeda, Sci. Rep. 10, 1239 (2020)