Low-temperature dielectric relaxation of molecular glasses:: Crossover from the nearly constant loss to the tunneling regime -: art. no. 174203

The dielectric loss of molecular glasses was investigated from the glass transition temperature T-g down to 2 K, for glycerol down to 0.1 K, in the frequency range 50 Hz-20 kHz. The systems studied here do not exhibit a beta process. Three distinct relaxation regimes are identified, namely nearly constant loss (NCL), thermally activated dynamics in asymmetric double well potentials (ADWP), and tunneling. The NCL (0.35 < T/T-g< 1), described by epsilon(')(nu)proportional to nu(-gamma) with gamma=0.09-0.21, exhibits the characteristic exponential temperature dependence. The ADWP dynamics (0.05 < T/T-g< 0.35) resembles that of silica. At lowest temperatures (2-6 K) the tunneling plateau is reached. This is associated with a power law spectrum revealing a universal positive exponent that agrees well with that reported for ionic, inorganic and polymeric glasses. The so-called plateau strength C=(2/pi)(.)tan delta=P mu(2)/(3 epsilon(0)epsilon(r)) is determined.