VARIATIONAL STUDY OF QUANTUM FLUCTUATIONS IN ANISOTROPIC ANTIFERROMAGNETS

The variational wave function proposed by Bartkowski to treat the grou nd state of a quantum antiferromagnet is generalized to arbitrary spin . We derive the fundamental equations of this method for the energy an d magnetization by using a local cluster expansion. The leading order corrections to the wave function in the limit of either large spin S o r lattice dimension D are identified. The good quality of the wave fun ction is confirmed by the ground state energy of E(0) = -0.3340J obtai ned for the Heisenberg S = 1/2 antiferromagnet on the square lattice. We present results obtained for the system of two planes and for three -dimensional SC and BCC lattices, as well as for the three-dimensional anisotropic Heisenberg model. It is found that the quantum fluctuatio ns in a two-plane system are already. closer to a three-dimensional th an to a two-dimensional antiferromagnet, while a weak interplane coupl ing in the latter case hardly influences the nearest-neighbour spin co rrelations within the planes.