EXTENDED SCHWINGER-BOSON THEORY OF THE 2-DIMENSIONAL SPIN-1 2 ANISOTROPIC HEISENBERG ANTIFERROMAGNETS/

An extended Schwinger-boson mean-field theory based on the symmetry co nsiderations is developed to study the zero-temperature properties of the two-dimensional anisotropic spin-1/2 Heisenberg antiferromagnets. The Bose condensation of the Schwinger bosons occurring in different f ashions at various anisotropies is shown to provide a natural descript ion of the variation of spin long range antialignment with the anisotr opy. The ground state energy and the excitation spectra are found cons istent with the numerical results both in the XY-like and the Ising-li ke anisotropic systems.