Response of a nonequilibrium system to a thermal perturbation

The nonequilibrtum statistical operator method is used to study the reaction of a nonequilibrium system to a small thermal perturbation described by a small correction to the entropy operator. In a linear approximation in the thermal perturbation, macroscopic equations are obtained that describe the evolution in time of a small deviation of the system from the unperturbed nonequilibrium process and general formulas are obtained for the linear response. The transport coefficients that occur in these equations are expressed in terms of the time correlation coefficients with respect to the unperturbed nonequilibrium state of the system. This state is described in terms of averaged macroscopic variables for the determination of which the corresponding balance equations are formulated. As an example, the formulas are constructed for the charge and heat fluxes that arise in a spatially homogeneous system of hot electrons in a semiconductor when a small temperature gradient is imposed.