COULD INPLANE EXCHANGE-ANISOTROPY INDUCE THE OBSERVED ANTIFERROMAGNETIC TRANSITIONS IN THE UNDOPED HIGH-TC MATERIALS

A tiny in-plane XY anisotropy-lambda = 0.02 could induce a substantially high Kosterlitz-Thouless phase transition at kT(c)/J = 0.25 in the 2D spin-1/2 antiferromagnet. Examination of transverse and longitudinal correlations indicates a Heisenberg to XY crossover near T(c), with a scaling relation suggesting an ordering transition at T(c) approximately 250 K for the anisotropies measured in the undoped high-T(c) copper oxides. The results also have implications for Kosterlitz-Thouless condensation in general 2D quantum systems.