High-Temperature Series Expansion for Complicated Level Systems

Abstract

Based on a Green's function formalism, a high-temperature series expansion technique applicable to systems with complicated energy levels is developed. The application of the method to magnetic systems with crystal fields is discussed. As an example, we have obtained the high-temperature series expansion of the susceptibility for a spin-1 ferromagnet with anisotropic exchange interactions and with a uniaxial crystal-field potential.