Nonlinear features in the dielectric behavior of propylene glycol

We investigate the nonlinear dielectric effects in a glass-forming polar liquid, propylene glycol, by high-voltage frequency-domain impedance experiments. The peak amplitudes of the sinusoidal electric fields are varied between 14 and 283 kV/cm. Two competing nonlinear effects are observed: a decrease of the dielectric constant reminiscent of the Langevin effect and an increase of the dielectric loss that originates from the irreversible transfer of energy from the electric field to the slow degrees of freedom of the viscous liquid. By virtue of the frequency dependence of the two features, the positive and negative changes are easily separable. Both effects can be rationalized quantitatively without adjustable parameters.