Evidence for coupling between charge density waves and phonons in two-dimensional rare-earth tritellurides

We report on a Raman-scattering investigation of the charge density wave (CDW), quasi-two-dimensional rare-earth tritellurides RTe₃ (R=La, Ce, Pr, Nd, Sm, Gd, and Dy) at ambient pressure, and of LaTe₃ and CeTe₃ under externally applied pressure. The observed phonon peaks can be ascribed to the Raman-active modes for both the undistorted and the distorted lattices in the CDW state by means of a first-principles calculation. The latter also predicts the Kohn anomaly in the phonon dispersion, driving the CDW transition. The integrated intensity of the two most prominent modes scales as a characteristic power of the CDW-gap amplitude upon compressing the lattice, which provides clear evidence for the tight coupling between the CDW condensate and the vibrational modes.