EQUIVALENCE BETWEEN SPIN-WAVES AND LATTICE BOSONS WITH APPLICATIONS TO THE HEISENBERG-MODEL

An equivalence theorem is proved between a spin wave system and its corresponding lattice boson system. We then apply the theorem to the anisotropic Heisenberg model in a magnetic field, but with a z-axis rotational symmetry; the model is characterized by two parametcrs, the magnetization M per site along the z-axis (which can vary from -1 to 1) and the anisotropicity A. We give two systematic expansions: One is valid for arbitrary A when 1 - M is small and the other holds for arbitrary 1 - M when A is small. © 1993 Academic Press, Inc.