Zero-bias anomalies and boson-assisted tunneling through quantum dots

We study resonant tunneling through a quantum dot with one degenerate level in the presence of strong Coulomb repulsion and a bosonic environment. Using a real-time diagrammatic formulation we calculate the spectral density and the nonlinear current. The former shows a multiplet of Kondo peaks split by the transport voltage and boson frequencies. This leads to zero-bias anomalies in the differential conductance, which shows a local maximum or minimum depending on the level position. We compare with recent experiments.