CUMULANT APPROACH TO THE LOW-TEMPERATURE THERMODYNAMICS OF MANY-BODY SYSTEMS

Current methods to describe the thermodynamic behavior of many-particl e systems are often based on perturbation theory with an unperturbed s ystem consisting of free particles. Therefore, only a few methods are able to describe both strongly and weakly correlated systems along the same lines. In this article we propose a cumulant approach which allo ws for the evaluation of excitation energies and is especially appropr iate to account for the thermodynamics at low temperatures. The method is an extension of a cumulant formalism which was recently proposed t o study statical and dynamical properties of many-body systems at zero temperature. The present approach merges into the former one for vani shing temperature. As an application we investigate the thermodynamics of the hole-doped antiferromagnetic phase in high-temperature superco nductors in the framework of the anisotropic t-J model.