Transport across an Anderson quantum dot in the intermediate coupling regime

We describe linear and nonlinear transport across a strongly interacting single impurity Anderson model quantum dot with intermediate coupling to the leads, i.e. with tunnel coupling Γ of the order of the thermal energy kBT. The coupling is large enough that sequential tunneling processes (second order in the tunneling Hamiltonian) alone do not suffice to properly describe the transport characteristics. Upon applying a density matrix approach, the current is expressed in terms of rates obtained by considering a very small class of diagrams which dress the sequential tunneling processes by charge fluctuations. We call this the “dressed second order” (DSO) approximation. One advantage of the DSO is that, still in the Coulomb blockade regime, it can describe the crossover from thermally broadened to tunneling broadened conductance peaks. When the temperature is decreased even further (kBT < Γ), the DSO captures Kondesque behaviours of the Anderson quantum dot qualitatively: we find a zero bias anomaly of the differential conductance versus applied bias, an enhancement of the conductance with decreasing temperature as well as universality of the shape of the conductance as function of the temperature. We can without complications address the case of a spin degenerate level split energetically by a magnetic field. In case spin dependent chemical potentials are assumed and only one of the four chemical potentials is varied, the DSO yields in principle only one resonance. This seems to be in agreement with experiments with pseudo spin [U. Wilhelm, J. Schmid, J. Weis, K.V. Klitzing, Physica E 14, 385 (2002)]. Furthermore, we get qualitative agreement with experimental data showing a cross-over from the Kondo to the empty orbital regime.