Suppression of the commensurate charge density wave phase in ultrathin 1T-TaS2 evidenced by Raman hyperspectral analysis

摘要

Using temperature-dependent and low-frequency Raman spectroscopy, we address the question of how the transition from bulk to a few atomic layers affects the charge density wave (CDW) phases in 1T-TaS2. We find that for crystals with thickness larger than approximate to 10 nm the transition temperatures between the different phases as well as the hysteresis that occurs in the thermal cycle correspond to the ones expected for a bulk sample. However, when the crystals become thinner than approximate to 10 nm, the low-temperature commensurate CDW phases can be suppressed down to the experimentally accessible temperatures (similar to 80 K) upon cooling at moderate rates (similar to 5 K min(-1)). In addition, even the near commensurate CDW phase is not accessible in few-layer flakes below approximate to 4 nm for even slower cooling rates (similar to 1 K min(-1)). We employ Raman hyperspectral imaging to statistically confirm these findings and consider the interlayer coupling and its dynamics to play significant roles in determining the properties of CDW systems consisting of a few unit cells in the vertical direction.