Heat Transport by Fracton Hopping in Amorphous Materials

The thermal conductivity $\kappa$ of epoxy resin, MY750, cured with the stoichiometric quantity and twice that quantity of ethylene diamine, has been measured in the range 2-110 K. Both specimens show a plateau in $\kappa$ in the liquid-helium region, and at higher temperatures $\kappa$ has an additional term which is proportional to the temperature. This is in accord with the model of Alexander, Entin-Wohlman, and Orbach [Phys. Rev. B 34, 2726 (1986)] for phonon-assisted fraction hopping above the plateau. The magnitude of this contribution, based on the theory of Jagannathan, Entin-Wohlman, and Orbach are in reasonable agreement with our experimental values.