Low-temperature dielectric relaxation of molecular glasses: Crossover from the nearly constant loss to the tunneling regime

The dielectric loss of molecular glasses was investigated from the glass transition temperature Tg 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 β 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∕Tg<1), described by ε^''(ν)∝ν^-γ with γ=0.09-0.21, exhibits the characteristic exponential temperature dependence. The ADWP dynamics (0.05<T∕Tg<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∕π)∙tan0.2em0exδ=Pμ²∕(3ε₀εᵣ) is determined.