Semi-analytical solution of the Kondo model in a magnetic field

The single impurity Kondo model at zero temperature in a magnetic field is solved by a semi-analytical approach based on the flow equation method. The resulting problem is shown to be equivalent to a resonant level model with a non-constant hybridization function. This nontrivial effective hybridization function encodes the quasiparticle interaction in the Kondo limit, while the magnetic field enters as the impurity orbital energy. The evaluation of static and dynamic quantities of the strong-coupling Kondo model becomes very simple in this effective model. We present results for thermodynamic quantities and the dynamical spin-structure factor and compare them with NRG calculations.