LOW-TEMPERATURE PROPERTIES OF A MODEL GLASS .2. SPECIFIC-HEAT AND THERMAL TRANSPORT

We study the low-temperature properties of glasses using the elastic dipole model introduced in the preceding paper (I). We show that harmonic excitations about the frozen ground states of the defect Hamiltonian dominate thermal properties in the 1e 10-K regime. We numerically determine the density of states for the defect modes from the simulation of I. The coupling of long-wavelength phonons to the defect modes is treated within perturbation theory, and is shown to lead to frequency-dependent softening of the medium and to strong phonon scattering in the terahertz frequency region. The defect modes account for the excess specific heat seen as the bump in C/T3. Resonant scattering of acoustic phonons off the defect modes leads to the plateau in the thermal conductivity. We compare our results with experiments on the orientational glass KBr:KCN and on vitreous silica. © 1990 The American Physical Society.