Signatures of XY behaviour in 2D weakly anisotropic antiferromagnets

We study the two-dimensional (2D) S = 1/2 Heisenberg antiferromagnet with weak easy-plane exchange anisotropy (down to 0.1%) by quantum Monte Carlo simulations. We observe a crossover between isotropic and XY behavior at a temperature which is similar to 30% above the Kosterlitz - Thouless transition. Evidences of the latter are hard to detect in real layered materials due to the interlayer exchange; however, below the crossover there exists a range where experimental detection of non-critical 2D XY behavior is possible, as shown in the case of Sr2CuO2Cl2