Identification of soft modes across the commensurate-to-incommensurate charge density wave transition in 1T-TaSe2

1T-TaSe2 is a prototypical charge density wave (CDW) material for which electron-phonon coupling and associated lattice distortion play an important role in driving and stabilizing the CDW phase. Here, we investigate the lattice dynamics of bulk 1T-TaSe2 using angle-resolved ultralow wavenumber Raman spectroscopy down to 10 cm-1. Our high-resolution spectra allow us to identify at least 27 Raman-active modes in the commensurate (CCDW) phase. Contrary to other layered materials, we do not find evidence of interlayer breathing or shear modes, suggestive of AA stacking in the bulk, or sufficiently weak interlayer coupling. Polarization dependence of the mode intensities allows assignment of their symmetry, which is supported by first-principles calculations of the phonons for the bulk structure using density functional theory. A detailed temperature dependence in the range T=80-500 K allows us to identify soft modes associated with the CDW superlattice. Upon entering the incommensurate (ICCDW) phase above 473 K, we observe a dramatic loss of resolution of all modes, and significant linewidth broadening associated with a reduced phonon lifetime as the charge-order becomes incommensurate with the lattice.