Theory of nonlinear nonequilibrium response

Linear response theory is extended to make it possible to treat nonlinear, and hence nonequilibrium, phenomena. Electric conductivity is discussed as an example of such nonlinear response, where a nonequilibrium distribution of quasiparticles might be established if an electric field of sufficient strength is applied. This theory is then applied to the Esaki effect in bismuth, a typical case of drastic nonlinear conduction. Numerically calculated results are in good agreement with experiments.