Zero-bias anomalies and boson-assisted transport through small quantum dots

We study resonant tunneling through a quantum dot with one degenerate state in the presence of a strong Coulomb repulsion and a bosonic environment. Using a real-time approach we calculate the spectral density and the nonlinear current within a conserving approximation. The spectral density shows a multiplet of Kondo peaks if the temperature and boson frequency are smaller than the intrinsic broadening of the dot state. As a consequence we find a zero-bias anomaly in the differential conductance which can show local maxima or minima dependent on the value of the gate voltage. The results are compared with recent experiments.