Nonlinear dielectric response of polar liquids.

The dielectric constant of a polar liquid in a strong electric field changes, in the lowest order, proportionally to the squared field magnitude. The proportionality coefficient (dielectric slope) is traditionally analyzed by mean-field models in terms of the saturation of alignment of individual dipoles as prescribed by the Langevin function. Only a decrease of the dielectric constant induced by the field is allowed by this model, in contrast to observations. Here, the dielectric slope is expressed in terms of the fourth-order cumulant of the liquid dipole moment. The cumulant is in turn separated into the two-particle dipolar correlations, expressed in terms of the Kirkwood factor and the liquid compressibility, and higher, triple and four-particle, dipolar correlations. The model allows both positive and negative dielectric slopes. Two-particle and higher-order correlations largely compensate each other. The analysis of experimental data suggests that dielectric slope gives experimental access to non-trivial triple and four-particle orientational correlations in polar liquids.