Low-temperature thermal expansion of disordered solids

The thermal-expansion coefficients of vitreous silica, neutron-irradiated vitreous silica, vitreous silica containing OH^- or K₂O, the amorphous polymers SC5 (Scotchcast 5 epoxy, 3M Company) and PMMA, (polymethylmethacrylate) the chalcogenide glass As₂S₃, glassy PdSiCu, and crystalline samples of Na -alumina and ZrO₂: Y₂O₃ have been measured to temperatures T1 K. The low-energy excitations generally found in glassy materials influence or dominate at T<1 K. However, unlike the specific heat and other properties, is not a universal property of glasses. The associated with the excitations may be large or small, positive or negative. The results may be explained provided the excitations exhibit a broad spectrum of deformation potentials in addition to a broad spectrum of energies.