Two-level systems and low-temperature glass dynamics: Spectral diffusion and thermal reversibility of hole-burning linewidths

Abstract

Intermediate time-scale time-dependent hole-burning measurements are reported on three glassy organic systems which undergo spectral diffusion: cresyl violet in ethanol at 1.30 and 2.13 K, resorufin in ethanol at 2.13 K, and resorufin in glycerol at 2.13 K. The hole width is observed to broaden on a log time scale from 0.1 to 5000 s for each ethanol system while no broadening is observed in the system of resorufin in glycerol. A detailed theoretical treatment is introduced which allows the raw data to be converted to the fluctuation rate distribution of the underlying modes responsible for dephasing. Using this theory, the broadening in ethanol is found to be the result of a distribution of glassy modes which is Gaussian on a log R scale with a center rate at ∼0.02 s−1. In addition, temperature cycling hole-burning results are reported on the system cresyl violet in ethanol. A hole is burned at 1.30 K and detected before, during, and after a temperature cycle to 2.13 K and back. The hole width is observed to broaden at the high temperature and then narrow again in a completely reversible manner when the temperature is again lowered. Theoretical calculations show this behavior to be entirely consistent with the tunneling two-level-system (TLS) model of glass dynamics but incompatible with other models such as particle or defect diffusion. The cycling data is shown to fall exactly on the theoretical curve calculated from the TLS model using no adjustable parameters.