Evidence for coupling between collective state and phonons in two-dimensional charge-density-wave systems

We report on a Raman scattering investigation of the charge-density-wave (CDW), quasi two-dimensional rare-earth tri-tellurides $R$Te$_3$ ($R$= La, Ce, Pr, Nd, Sm, Gd and Dy) at ambient pressure, and of LaTe$_3$ and CeTe$_3$ under externally applied pressure. The observed phonon peaks can be ascribed to the Raman active modes for both the undistorted as well as the distorted lattice 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

TORSIONS AROUND THE THE SINGLE BOND IN CONJUGATED MOLECULES

Author Institution: Mellon InstituteThis paper deals with torsions around the single bond in molecules containing the group $=C-C=$. Butadiene $(CH_{2}=CH-CH=CH_{2})$ and glyoxal (OCH-CHO) are prototypes. Benzaldehyde is included as an example of the system [FIGURE]-C=. Torsional frequencies have been observed between 40 and $170 cm^{-1}$ for a dozen such compounds in the vapor state. The evaluation of a potential function from the infrared frequency is difficult, if not impossible. If it is written as the usual series [FIGURE] at least two of the three parameters $V_{1}, V_{2}$, and $V_{3}$ are important for the butadiene and glyoxal derivatives. Since there is only one experimental datum, the dilemma is obvious. Nevertheless some interesting comparisons can be made between compounds. For benzaldehyde, $V_{1}$ and $V_{2}$ are zero by symmetry and $V_{2}$ can be evaluated. The barrier for benzaldehyde is very different in the gas and the liquid states