Phonons and Thermal Properties of bcc and fcc Helium From a Self-Consistent Anharmonic Theory

Numerical calculations of phonon spectra, including damping, are reported for bcc He-4 and He-4 and for fcc He-4. Strong damping is found for the longitudinal branches near the boundary of the Brillouin zone. In the bcc phase anomalous dispersion occurs for several directions at long wavelengths, which is most pronounced in the lowest transverse branch in (110) direction. This leads to an anomaly in the specific heat at low temperatures. In this calculation anharmonicities and short-range correlations are treated in a self-consistent way.