Magnetic order in the quasi-two-dimensional easy-plane XXZ model

A Green's-function theory of antiferromagnetic short-range and long-range order (LRO) in the $S=1/2$ quasi-two-dimensional easy-plane XXZ model is presented. As the main new result, {\it two} phase transitions due to the combined influence of spatial and spin anisotropy are found, where below the higher and lower N\'{e}el temperature there occurs LRO in the transverse and in both the transverse and longitudinal spin correlators, respectively. Comparing the theory with neutron-scattering data for the correlation length of $\rm La_2CuO_4$, a very good agreement in the whole temperature dependence is obtained. Moreover, for $\rm La_2CuO_4$, $\rm Sr_2CuO_2Cl_2$, and $\rm Ca_{0.85}Sr_{0.15}CuO_2$ the second phase with longitudinal LRO is predicted to appear far below room temperature.