| We consider a single-level quantum dot in the presence of strong Coulomb interaction under nonequilibrium condition. Making use of an equations-of-motion technique generalized out-of-equilibrium and developed in successive orders in the tunnel-coupling strength, we study the transport behavior of the system driven out of equilibrium by either (i) a time-independent external field (application of a dc bias voltage) or (ii) a time-dependent external field (fast switching of the gate voltage). In the former case (i) when a dc bias voltage V is applied to the leads, we focus on the properties of the system in its steady state and compute the transition rates, the spectral density and the non-linear I-V characteristics. We find a Zero-Bias-Anomaly (ZBA) in the V-dependence of the differential conductance followed by a broad Coulomb peak at large V. The low-bias differential conductance is found to be a universal function of the normalized bias voltage V/TK, where TK is the Kondo temperature. The universal scaling with a single energy scale TK at low bias voltage is also observed for the normalized transition rates and controls the crossover of the system from the strong to the weak coupling regime when either temperature or V is increased. Finally in the case (ii) when the system is submitted to a fast switching of the gate voltage, we study the quantum dynamics of spin and charge in the transient regime. We find an exponential relaxation behavior of charge and spin which is respectively governed by a single time scale each defining the relaxation time. We draw the evolution of the corresponding charge and spin relaxation rates as a function of the dot level position and Coulomb interaction and show how they differ from each other due to Coulomb interaction. |
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