Origin of Stripe and Quasi-Stripe CDW Structures in Monolayer MX$ _2$ compounds: Multivalley Free Energy Landscape

摘要: Ultrathin sheets of transition metal dichalcogenides (MX$ _2$) with charge density waves (CDWs) is increasingly gaining interest as a promising candidate for graphene-like devices. Although experimental data including stripe/quasi-stripe structure and hidden states have been reported, the ground state ultrathin MX$ _2$ compounds and, in particular, origin anisotropic (stripe quasi-stripe) CDW phases long-standing problem. Anisotropic explained by Coulomb interaction between domain walls inter-layer interaction. However, these models assume that can exist first place. Here, we report appear naturally without assuming interactions: We explain topological defect theory (line defects two-dimensional plane) interference harmonics macroscopic wave functions. revisit McMillan-Nakanishi-Shiba model monolayer 1$T$-TaS$ 2$H$-TaSe$ show CDWs vectors are separated $120^\circ$ (i.e. three-fold rotation symmetry underlying lattice) contain free-energy landscape many local minima. Then, remove this constraint free energy minima corresponding to stripe quasi-stripe phase appear. Our results imply may be secondary factors appearance phases. Furthermore, predict new phases, hence it become basis study further. anticipate our starting point further physics, such explanation "Hidden states", interplay supersolid lattice symmetry, application other van der Waals structures.