Correlation Theory of Planar Magnets forT>~Tc

Abstract
A self-consistent theory for static and dynamic properties of planar magnets including quantum and correlation effects is presented. The renormalization and damping of the potentially soft mode is calculated. A central peak absorbing the spectral weight near the transition temperature is found. Qualitative agreement is obtained with observations made on Pr, which approximately represents a singlet-doublet model. The correlation theory is a systematic generalization of the random-phase approximation with an equally wide range of applicability.