Probing heterogeneous thermal relaxation by nonlinear dielectric spectroscopy

Abstract

The application of time-dependent electric fields to a sample that exhibits dielectric lossresults in the irreversible transfer of energy from the external field to the slow degrees offreedom in the material. These slow modes are coupled only weakly to the phonon bathand elevated fictive temperatures are thus associated with considerable persistencetimes. Assuming locally correlated heterogeneities regarding dielectric and thermalrelaxation times, extremely pronounced nonlinear dielectric effects are predicted. Fortwo glass-forming systems, glycerol and propylene glycol, the predicted effectsare observed experimentally by high-field impedance spectroscopy. At fields of283 kV cm−1, the dielectric loss increases by up to 9% over its low-field value. This nonlinearity displaysa characteristic frequency dependence, with the loss at frequencies below the peakvalue being field invariant, whereas the high-frequency wing experiences a nearuniform relative increase of the loss. If the dielectric and thermal time constants areassumed to be independently distributed, the model fails to explain the findings.