Spectral function of the Kondo model in high magnetic fields -: art. no. 014430

Using a recently developed perturbative renormalization group (RG) scheme, we calculate analytically the spectral function of a Kondo impurity for either large frequencies omega or large magnetic field B and arbitrary frequencies. For large omega>max[B,T-K] the spectral function decays as 1/ln(2)[omega/T-K] with prefactors which depend on the magnetization. The spin-resolved spectral function displays a pronounced peak at omegasimilar toB 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(omega,B)/T-K]. Furthermore, we assess the ability of the NRG to resolve structures at finite frequencies.