Field dependent quasiparticles in a strongly correlated local system

We show how quasiparticles in a magnetic field of arbitrary strength H can be described by field dependent parameters. We illustrate this approach in the case of an Anderson impurity model and use the numerical renormalization group (NRG) to calculate the renormalized parameters for the levels with spin sigma, epsilon(d,sigma)(H), resonance width Delta(sigma)(H), and the effective local quasiparticle interaction U(H). In the Kondo or strong correlation limit of the model the progressive de-renormalization of the quasiparticles can be followed as the magnetic field is increased. The low temperature behavior, including the conductivity, in arbitrary magnetic field can be calculated in terms of the field dependent parameters using the renormalized perturbation expansion. Using the NRG the field dependence of the spectral density on higher scales is also calculated.