Could in-plane exchange anisotropy induce the observed antiferromagnetic transitions in the undoped high-Tcmaterials?

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