Symmetry Breaking and Charge Density Wave Instabilities in Ultra-Thin Two-Dimensional Chalcogenides

Two-dimensional materials have shown complete paradigm shift for the understanding of physical and electronic properties because of reduced dimensionality, symmetry breaking and quantum confinement effects. The reduced quantum size strongly affects the electron correlations, which are important to understand the many body physics of electron-electron and electron-phonon interactions. [1,2] Charge density wave is a low temperature strongly correlated ordered phase that arises from periodic modulation of atomic lattice accompanying with the modulations on the electronic charge density.[3] Charge density wave is exclaimed to be driven from Peierls distortion, where electronic instabilities are results from Fermi-surface nesting.[4] However, in the cases of several layered two-dimensional materials, NbSe₂, VSe₂, TaS₂, TaSe₂, nesting alone is insufficient to explain the charge density wave. For instance, strong electron-phonon coupling plays a major important role than Fermi-surface nesting. Charge density wave is mostly understood for bulk, but for atomically thin materials, where the confinement effects dominate, the onset of charge density wave is unclear. Our low temperature optical study on the thin flakes of 2H-TaS₂ suggest that the charge density wave can persist even in the ultra-thin layer (~ 3 nm) and transition temperature (T_ICDW) well above ~ 200 K, which is quite high as compared to bulk 2H-TaS₂ (~ 76 K).[5] The understanding of electron-phonon coupling, structural instabilities, tuning of charge density wave transition temperature with reduced dimension can be inferred using low temperature Raman spectroscopy and charge transport measurements.<br/><b>References</b><br/>[1] D. Rawat, A. Singh, N. K. Singh, and A. Soni, Anisotropic light-matter interactions in the single-crystal topological insulator bismuth selenide. Phys. Rev. B 107, 155203 (2023).<br/>[2] J. Zhang, Z. Peng, A. Soni, Y. Zhao, Y. Xiong, B. Peng, J. Wang, M. S. Dresselhaus, and Q. Xiong, Raman Spectroscopy of Few-Quintuple Layer Topological Insulator Bi2Se3 Nanoplatelets. Nano Lett. 11, 2407 (2011).<br/>[3] J. Pandey and A. Soni, Electron-phonon interactions and two-phonon modes associated with charge density wave in single crystalline VSe2. Phys. Rev. Res. 2, 033118 (2020).<br/>[4] G. Grüner, The dynamics of charge-density waves. Rev. of Mod. Phys. 60, 1129 (1988).<br/>[5] D. Rawat, A. Thomas, A. P. Singh Rana, C. Bera, and A. Soni, Symmetry breaking and structural instability in ultrathin 2HTaS2 across the charge density wave transition. Phys. Rev. B 109, 155411 (2024).