On a Connection Between Nonlinear Response to an External Field and Equilibrium Properties of Systems with Interaction: Comment on cond-mat/9711226 by Kopietz and Voelker

We demonstrate the danger of a naive identification of kinetic coefficients in the free-electron response expansion in powers of an external electric field and coefficients in the expansion of the corresponding equilibrium quantity in powers of the fluctuating intrinsic electric field. A particular example of the direct current generation by an AC electric field in a mesoscopic system of non-interacting electrons vs. the Fock contribution to the equilibrium persistent current in a system of interacting electrons is considered.