Transport through quantum dots with large level spacing and charging energy is considered. At low temperature and strong coupling to the leads, quantum fluctuations and the Kondo effect become important. They show up, e.g., as zero-bias anomalies in the current-voltage characteristics. We use a recently developed diagrammatic technique as well as a new real-time renormalization-group approach to describe charge and spin fluctuations. Both approaches cover the linear as well as the nonlinear response regime. The spin fluctuations give rise to a Kondo-assisted enhancement of the current through the dot as seen in experiments.
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