Dissipation in nonlinear response

It is shown that the measurable nonlinear susceptibilities of third and higher orders probe only a fraction of equilibrium fluctuations, and in general can give only lower bounds on the equilibrium correlation functions. This contrasts with the linear and quadratic response, which completely determines the corresponding correlations by general fluctuation–dissipation theorems. The exactly solvable one‐dimensional kinetic Ising model illustrates which fluctuations give rise to dissipation; only two‐m a g n o n processes of short wavelength and opposite momenta contribute to the third‐order nonlinear susceptibilities, while two‐ and three‐m a g n o n processes contribute to the corresponding equilibrium correlation functions.