Probing heterogeneous thermal relaxation by nonlinear dielectric spectroscopy

The application of time-dependent electric fields to a sample that exhibits dielectric loss results in the irreversible transfer of energy from the external field to the slow degrees of freedom in the material. These slow modes are coupled only weakly to the phonon bath and elevated fictive temperatures are thus associated with considerable persistence times. Assuming locally correlated heterogeneities regarding dielectric and thermal relaxation times, extremely pronounced nonlinear dielectric effects are predicted. For two glass-forming systems, glycerol and propylene glycol, the predicted effects are observed experimentally by high-field impedance spectroscopy. At fields of 283 kV cm^-1, the dielectric loss increases by up to 9% over its low-field value. This nonlinearity displays a characteristic frequency dependence, with the loss at frequencies below the peak value being field invariant, whereas the high-frequency wing experiences a near uniform relative increase of the loss. If the dielectric and thermal time constants are assumed to be independently distributed, the model fails to explain the findings.