Magnetic order-disorder transition in the two-dimensional spatially anisotropic Heisenberg model at zero temperature

The ground-state properties of the spin-1/2 antiferromagnetic Heisenberg model with spatially anisotropic couplings on a square lattice are investigated by a spin-rotation-invariant Green's-function approach and by Lanczos diagonalizations on lattices up to 36 sites supplemented by finite-size scaling. We focus on the anisotropy-driven transition from the Neel state to a paramagnetic state with antiferromagnetic short-range order and on the spatial dependence of spin correlation functions. Out principal result is that a rather sharp crossover in the magnetic behavior occurs at the coupling ratio R(0)similar or equal to 0.2(R=J(y)/J(x)). [S0163-1829(99)03537-7].