Fluctuations, escape, and nucleation in driven systems: Logarithmic susceptibility

We analyze the probabilities of large infrequent fluctuations in nonadiabatically driven systems. In a broad range of the driving field magnitudes, the logarithm of the fluctuation probability is linear in the field, and the response can be characterized by a logarithmic susceptibility (LS). We evaluate the activation energies for nucleation, with account taken of the field-induced lifting of time and spatial degeneracy of instantonlike nucleation trajectories. LS for nucleation in systems with nonconserved order parameter is shown to be a nonmonotonic function of ω and k.