Kondo effect in real metals

Abstract

Starting from the most general form of the Anderson hamiltonian, the behaviour of magnetic impurities in real metals is considered, taking into account the orbital structure of the local impurity electrons, crystal field and spin orbit splittings. The analysis is carried out in an atomic limit, in which the impurity has a well defined integer valency (a Schrieffer Wolff transformation is then valid). The main steps of a scaling procedure are described in detail. As the temperature goes down, the excited states of the ground state configuration decouple one after the other. The hierarchy of these decouplings, and their interplay with Kondo singularities are analyzed. When a Fermi liquid picture applies as T → 0, the number of independent parameters may be reduced considerably using symmetry and universality arguments which bypass the numerical description of the crossover region. That first part sets a language in which to describe specific problems. We apply that language to the case where the atomic ground state is an orbital singlet. In the absence of anisotropies, the only parameters are the impurity spin S and the number of orbital channels n. We show that an anomalous fixed point occurs at finite coupling when n > 2 S. That fixed point is unstable with respect to anisotropies. The scaling trajectories are discussed for a cubic crystal field for several choices of valencies. The universality of the low temperature behaviour is clarified. A similar analysis is carried out when the atomic ground state only has one electron (or hole). The influence of crystal field and spin orbit interactions is analyzed — and their relevance to the Kondo crossover and to universality is ascertained.