Abstract
The low-temperature thermal properties of an amorphous polymer were used to probe glassy behavior as a function of pressure. Specifically, the thermal diffusivity scra and thermal conductivity κ of an epoxy were measured over the range 0.3–10 K at pressures up to roughly 4 kbar. The specific heat determined from κ/scra was observed to decrease with pressure; the relative changes were rather uniform over the entire temperature range. The thermal-conductivity measurements revealed an increased conductivity with pressure for temperatures above 1 K. The magnitude of this pressure-induced increase in κ was smaller at lower temperatures; near 0.3 K, the measurements indicated decreased κ as pressure was applied. Analysis of these measurements using the tunneling model suggests that the energy density of two-level excitations decreases with pressure, while the coupling of these excitations to phonons increases. The measured changes in the 0.3–1-K regime indicate that the density of two-level systems depends on the phonon velocity v as . The magnitude of the pressure-induced changes in the range 1–10 K suggests that the strong phonon scattering and excess excitations in this regime are most likely not related to structural length scales in the glass. Finally, the similar changes with pressure over the entire temperature range suggest that all the excitations, namely phonons, two-level systems, and the additional modes above 1 K, are related. These results are discussed with regard to the tunneling model, microscopic models, and the fracton theory.
- Received 26 January 1989
DOI:https://doi.org/10.1103/PhysRevB.40.1901
©1989 American Physical Society