RESONANT-TUNNELING THROUGH ULTRASMALL QUANTUM DOTS - ZERO-BIAS ANOMALIES, MAGNETIC-FIELD DEPENDENCE, AND BOSON-ASSISTED TRANSPORT

We study resonant tunneling through a single-level quantum dot in the presence of strong Coulomb repulsion beyond the perturbative regime. T he level is either spin degenerate or can be split by a magnetic field . Furthermore we discuss the influence of a bosonic environment. Using a red-time diagrammatic formulation, we calculate transition rates, t he spectral density, and the nonlinear I-V characteristic. The spectra l density shows a multiplet of Kondo peaks split by the transport volt age and the boson frequencies and shifted by the magnetic field. This leads to zero-bias anomalies in the differential conductance, which ag ree well with recent experimental results for the electron transport t hrough single-charge traps. Furthermore, we predict that the sign of t he zero-bias anomaly depends on the level position relative to the Fer mi level of thr leads.