Renormalization-group analysis of long-range order in the two-dimensional antiferromagnetic Heisenberg model

Abstract

We perform a real-space renormalization-group analysis for the two-dimensional antiferromagnetic Heisenberg model. We first derive an effective model, described by the Hamiltonian: H=J ${\mathcal{J}}_{\mathit{j}=1}^{\mathit{L}}$ S(j)\ensuremath{\cdot}S(j+1)+ ${\mathcal{J}}_{\mathit{j}=1}^{\mathit{L}}$(-1${)}^{\mathit{j}}$\ensuremath{\Vert}J(j)\ensuremath{\Vert}S(0)\ensuremath{\cdot}S(j), where S(0) represents a single spin located in the center of the ring. We show that the renormalized coupling constants \ensuremath{\Vert}J(j)\ensuremath{\Vert} tend to nonzero values as one increases the block size by successive renormalization. Using analytic and numerical arguments, we establish the existence of antiferromagnetic long-range order in the effective model and hence, in the original Heisenberg model. We compare our results with those obtained from perturbation theory, spin-wave theory, and exact-diagonalization calculations.