Spectral function of the Kondo model in high magnetic fields

Using a recently developed perturbative renormalization group (RG) scheme, we calculate analytically the spectral function of a Kondo impurity for either large frequencies w or large magnetic field B and arbitrary frequencies. For large ω»max[B,T K ] the spectral function decays as 1/ln 2 [ω/T K ] with prefactors which depend on the magnetization. The spin-resolved spectral function displays a pronounced peak at ω∼B with a characteristic asymmetry. In a detailed comparison with results from numerical renormalization group (NRG) and bare perturbation theory in next-to-leading logarithmic order, we show that our perturbative RG scheme is controlled by the small parameter 1/ln[max(ω,B)/T K ]. Furthermore, we assess the ability of the NRG to resolve structures at finite frequencies.