Real-space renormalization group study of the anisotropic antiferromagnetic Heisenberg model on a 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 cr itical value of Delta>0, at which the critical temperature goes to zero, an d the presence of reentrant behavior on the critical line between the order ed and disordered phases. This whole critical line is found to belong to th e same universality class as the lsing model. Our results are in accordance with previous RSRG approaches but not with numerical simulations and spin- wave calculations.