The 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-neighbour couplings J and J'. On increasing J' > J the model shows in the isotropic Heisenberg limit a second-order transition from semi-classical Neel order to a quantum paramagnetic phase with enhanced local dimer correlations on the P bonds at about J(c)(') similar to 2.5 (...) 3J. This transition is driven by the quantum competition between J' and J. On increasing the anisotropy parameter Delta > 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(c)(') with Delta, i.e. the transition disappears in the Ising limit Delta --> infinity.