Influence of Ising - anisotropy on the zero - temperature phase transition in the square lattice spin - 1 2 J − J ′ model

We use a variational mean-field like approach, the coupled cluster method (CCM) and exact diagonalization to investigate the ground-state order-disorder transition for the square lattice spin-half XXZ model with two different nearest-neighbor couplings J and J ′. Increasing J ′ > J the model shows in the isotropic Heisenberg limit a second-order transition from semi-classical Néel order to a quantum paramagnetic phase with enhanced local dimer correlations on the J ′ bonds at about J ′ c ∼ 2.5. .. 3J. This transition is driven by the quantum competition between J ′ and J. Increasing the anisotropy parameter ∆ > 1 we diminish the quantum fluctuations and thus the degree of competition. As a result the transition point J ′ c is shifted to larger values. We find indications for a linear increase of J