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Two Phonon Interactions and Charge Density Wave in Single Crystalline VSe2 Probed by Raman Spectroscopy

Charge density wave (CDW) is a unique phenomenon mostly realized in two-dimensional (2D) metallic layered transition metal dichalcogenides (TMDCs). Here, we report on Raman spectroscopy of single crystal 1T-VSe2 and observed signature of commensurate CDW (C-CDW) and incommensurate (I-CDW) transition, in the temperature range of 50-120 K. The room temperature Raman spectra showed a sharp A1g mode ~ 206 cm-1 along with two new weak and broad Raman modes associated to Eg mode ~ 257 cm-1 and two-phonon mode (2ph) ~ 332 cm-1. The onset of I-CDW and C-CDW is estimated from resistance measurements supported by magnetic measurements. Remarkably, at the onset of I-CDW ~ 115 K, a significant enhancement in the intensity of weak Eg mode is observed along with emergence of a doubly degenerate Eg(2) mode ~ 144 cm-1. Below 70 K, a weak A1g mode ~ 170 cm-1 emerged signifying the onset of C-CDW. Anomalous phonon softening of Eg mode ~ 257 cm-1 and 2ph process mode below I-CDW has been also observed, which is signifying the involvement of electron-phonon coupling.