Theory of Static and Dynamic Properties of Crystal Field Systems Including Correlation Effects

There have been relatively few attempts to calculate the damping and renormalization of the excitations in a general crystal field system. For systems with the Ruderman Kittel exchange interaction the damping induced by the coupling to the conduction electrons was considered by Fulde et al.1. The effects of such couplings to other excitations will not be treated in this paper.

The renormalization and damping caused by the mutual interactions between the crystal field excitations, which is the subject here, was considered by perturbation theory in 1/z (z being the number of nearest neighbors) by Yang and Wang2 and applied to the singlet-doublet case by Bak3. For a general system this approach is cumbersome, and since it is not selfconsistent it is not applicable close to the transition temperature Tc.