Elektronentransport durch Nanostrukturen
While the properties of the Kondo model in equilibrium are well understood, much less is known for Kondo systems out of equilibrium. In this talk the properties of a quantum dot in the Kondo regime will be discussed, specifically when a large bias voltage V and/or a large magnetic field B is applied. Using the perturbative renormalization group generalized to stationary nonequilibrium situations, the renormalized couplings may be calculated, keeping their important energy dependence. We show that in a magnetic field the spin occupation of the quantum dot is nonthermal, being controlled by V and B in a complex way to be calculated by solving a quantum Boltzmann equation. We find that the well known suppression of the Kondo effect at finite V >> T_K (Kondo temperature) is caused by inelastic dephasing processes induced by the current through the dot. We calculate the corresponding decoherence rate, which serves to cut off the RG flow usually well inside the perturbative regime. As a consequence, the differential conductance, the local magnetization, the spin relaxation rate and the local spectral function may be calculated for V,B >> T_K in a controlled way. Experimental data are shown to be explained well by our theory.