We study resonant tunneling through a single-level quantum dot in thepresence of strong Coulomb repulsion beyond the perturbative regime. The levelis either spin-degenerate or can be split by a magnetic field. We, furthermore,discuss the influence of a bosonic environment. Using a real-time diagrammaticformulation we calculate transition rates, the spectral density and thenonlinear $I-V$ characteristic. The spectral density shows a multiplet of Kondopeaks split by the transport voltage and the boson frequencies, and shifted bythe magnetic field. This leads to zero-bias anomalies in the differentialconductance, which agree well with recent experimental results for the electrontransport through single-charge traps. Furthermore, we predict that the sign ofthe zero-bias anomaly depends on the level position relative to the Fermi levelof the leads.
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