Nonequilibrium dynamics of the Anderson impurity model

The M-channel Anderson impurity model (M = 1,2) is studied in the Kondo limit with a finite voltage bias applied to the conduction-electron reservoirs. Using the noncrossing approximation (NCA), we calculate the local spectral functions, the differential conductance, and susceptibility at nonzero bias for symmetric as well as asymmetric coupling of the impurity to the leads. We describe an effective procedure to solve the NCA integral equations that enables us to reach temperatures far below the Kondo scale; This allows us to study the scaling regime where the conductance depends on the bias only via a scaling function. Our results are applicable to both tunnel junctions and to point contacts. We present a general formula that allows one to go between the two cases of tunnel junctions and point contacts. Comparison is also made between the conformal field theory and the NCA conduction-electron self-energies in the two-channel case.