Reals-space renormalization group study of the anisotropic   antiferromagnetic Heisenberg model on the square lattice
  In this work we apply two different real-space renormalization-group (RSRG) approaches to the anisotropic antiferromagnetic spin-1/2 Heisenberg model on the square lattice. Our calculations allow for an approximate evaluation of the $T$ vs. $\Delta$ phase-diagram: the results suggest the existence of a critical value of $\Delta>0$, at which the critical temperature goes to zero, and the presence of reentrant behavior on the critical line between the ordered and disordered phases. This whole critical line is found to belong to the same universality class as the Ising model. Our results are in accordance with previous RSRG approaches but not with numerical simulations and spin-wave calculations. 
N. S. Branco and J. R. de Sousa