Inelastic Neutron Scattering in Epoxy Resins: The Phonon-Fracton Density of States and the Specific Heat

Inelastic neutron-scattering experiments have been used to determine the density of states, $G(E)$, of thermal excitations in epoxy resins. The results show that, over part of the range, $G(E)$ increases with an energy (E) dependence which is higher than E². This is in accord with the predictions of the phonon-fracton model of Derrida, Orbach, and Wu. Calculations of the low-temperature specific heat using the measured density of states are in reasonable agreement with experiment, but it is shown that these calculations are very sensitive to the values of $G(E)$ below $E=1$ meV.