ISING EXPANSION FOR THE HUBBARD-MODEL

We develop series expansions for the ground state properties of the Hu bbard model by introducing an Ising anisotropy into the Hamiltonian. F or the two-dimensional square lattice half-filled Hubbard model, the g round state energy, local moment, sublattice magnetization, uniform ma gnetic susceptibility, and spin stiffness are calculated as a function of U/t, where U is the Coulomb constant and t is the hopping paramete r. Magnetic susceptibility data indicate a crossover around U approxim ate to 4 between spin density wave antiferromagnetism and Heisenberg a ntiferromagnetism. Comparisons with Monte Carlo simulations, random ph ase approximation result, and mean-field solutions are also made.