Nonlinear response of a Kondo system: Perturbation approach to the time-dependent Anderson impurity model

Nonlinear tunneling current through a quantum dot (an Anderson impurity system) subject to both constant and alternating electric fields is studied in the Kondo regime. A systematic diagram technique is developed for perturbation study of the current in physical systems out of equilibrium governed by time-dependent Hamiltonians of the Anderson and the Kondo models. The ensuing calculations prove to be too complicated for the Anderson model, and hence, a mapping on an effective Kondo problem is called for. This is achieved by constructing a time-dependent version of the Schrieffer-Wolff transformation. Perturbation expansion of the current is then carried out up to third order in the Kondo coupling J yielding a set of remarkably simple analytical expressions for the current. The zero-bias anomaly of the direct current (dc) differential conductance is shown to be suppressed by the alternating field while side peaks develop at finite source-drain voltage. Both the direct component and the first harmonics of the time-dependent response are equally enhanced due to the Kondo effect, while amplitudes of higher harmonics are shown to be relatively small. A "zero-alternating-bias anomaly" is found in the alternating current (ac) differential conductance, that is, it peaks around zero alternating. bias. This peak is suppressed by the constant bias. No side peaks show up in the differential alternating conductance but their counterpart is found in the derivative of the ac with respect to the direct bias. The results pertaining to nonlinear response are shown to be valid also below the Kondo temperature.