The Nature of Glass Dynamics: Thermal Reversibility of Spectral Diffusion in a Low Temperature Glass

Temperature cycling optical hole-burning experiments on cresyl violet in ethanol glass are reported. At 1.3 K a hole is burned and measured. The temperature if then raised to 2.1 IQ the hole is measured. The temperature is returned to I, 3 K, and the hole is again measured. The hole is observed to be narrow at 1.3 K, broader at 2.1 K, and then narrow again when the temperature is returned to 1.3 IL. Using the time-dependent hole widths and photon echo data at the two temperatures, the results can be accounted for quantitatively using the tunneling two-level system model of glasses. Models of glasses, which involve processes such as spatial diffusion of defects, cannot account for the data.