Distinct surface and bulk charge density waves in ultrathin 1T-TaS2
  We employ low-frequency Raman spectroscopy to study the nearly commensurate (NC) to commensurate (C) charge density wave (CDW) transition in 1T-TaS2 ultrathin flakes protected from oxidation. We identify new modes originating from C phase CDW phonons that are distinct from those seen in bulk 1T-TaS2. We attribute these to CDW modes from the surface layers. By monitoring individual modes with temperature, we find that surfaces undergo a separate, low-hysteresis NC-C phase transition that is decoupled from the transition in the bulk layers. This indicates the activation of a secondary phase nucleation process in the limit of weak interlayer interaction, which can be understood from energy considerations. 
Rui He, Junichi Okamoto, Zhipeng Ye, Gaihua Ye, Heidi Anderson, Xia   Dai, Xianxin Wu, Jiangping Hu, Yu Liu, Wenjian Lu, Yup...
cond-mat.mtrl-sci