Theoretical studies of the phase transition in the anisotropic two-dimensional square spin lattice -: art. no. 094415

The phase transition occurring in a square two-dimensional spin lattice governed by an anisotropic Heisenberg Hamiltonian has been studied according to two recently proposed methods. The first one, the Dressed Cluster Method, provides excellent evaluations of the cohesive energy. The discontinuity of its derivative around the critical (isotropic) value of the anisotropy parameter confirms the first-order character of the phase transition. Nevertheless, the method introduces two distinct reference functions (either Neel or XY) which may, in principle, force the discontinuity. The Real-Space Renormalization Group with Effective Interactions does not reach the same numerical accuracy, but it does not introduce a reference function and the phase transition appears qualitatively, due to the existence of two domains with specific fixed points. The method confirms the dependence of the spin gap on the anisotropy parameter occurring in the Heisenberg-Ising domain.