Electron-phonon interactions and two-phonon modes associated with charge density wave in single crystalline 1 T-v Se2

Abstract

The charge density wave (CDW) is a unique phenomenon mostly realized in two-dimensional (2D) metallic layered transition metal dichalcogenides. We report on the observation of commensurate CDW (C-CDW) and incommensurate (I-CDW) transition in single crystal 1T-VSe2 using the spectroscopy technique in the temperature range of 50-120 K. The room temperature Raman spectra showed a sharp A1g mode ∼206cm-1 along with two additional modes associated to Eg∼257cm-1 symmetry and two-phonon (2ph)∼332cm-1 interactions. The onset temperature 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 the weak Eg mode is observed along with emergence of a doubly degenerate Eg(2) mode ∼144cm-1, while below 70 K, the emergence of the A1g mode ∼170cm-1 signifies the onset of C-CDW. The observation of a sudden rise in the intensity of the Eg mode ∼257cm-1 mode, below I-CDW, showed the involvement of electron-phonon coupling in 1T-VSe2.