Lattice Vibrations in Glasses.

I describe an experimental study of the lattice vibrations of glasses, predominantly through measurements of thermal conductivity in the temperature range 30 to 300 K. Crucial to this study was the development of a new A.C. technique, the 3omega method, which is insensitive to errors from black-body radiation and can be used to measure bulk glasses and crystals as well as amorphous films tens of microns thick. The data for a wide variety of amorphous solids agree with a model of the lattice vibrations first proposed by Einstein in 1911. In this model, the vibrational modes of the solid have short lifetimes, or in other words, the oscillations are strongly damped by neighboring oscillations. Making the assumption that each vibrational mode of a Debye solid lives for only one-half a period of vibration leads to good agreement with the data near room temperature. The model can be extended to the lattice vibrations of heavily disordered crystals that display glass-like properties at low temperatures. I studied a new example of a crystal with glass-like properties, Ba_ {1 - x}La_{x} F_{2 + x}, because the degree of disorder can be varied continuously from x = 0 to x = 0.50 and isolated defects at x < 0.01 have little influence on the thermal conductivity. Near x = 0.50, the thermal conductivity of Ba_ {1 - x}La_{x} F_{2 + x} is nearly identical to that of glasses from 0.1 to 300 K and agrees with the model near room temperature. Using Ba_{1 - x}La _{x}F_{2 + x}, I explore questions of the origin and universal density of low-energy excitations in glasses and disordered crystals. While all disordered crystals that display glass-like properties below 1 K also show strong phonon scattering near room temperature, I cannot draw a direct connection between the high and low temperature properties. A novel internal friction technique was developed to study the connection between low-energy excitations below 1 K and thermally activated relaxations at higher temperatures. The internal friction of glasses and disordered crystals have a wide range of behaviors above 50 K; in particular, the gaussian distribution of barrier heights observed in (KBr)_{1 - x}(KCN) _{x} is not duplicated in any other disordered crystal or in amorphous solids.