A Plane of Weakly Coupled Heisenberg Chains: Theoretical Arguments and   Numerical Calculations
  The $S=1/2$, nearest-neighbor, quantum Heisenberg antiferromagnet on the square lattice with spatially anisotropic couplings is reconsidered, with particular attention to the following question: at T=0, does N\'eel orderdevelop at infinitesimal interchain coupling, or is there a nonzero critical coupling? A heuristic renormalization group argument is presented which suggests that previous theoretical answers to that question are incorrect or at least incomplete, and that the answer is not universal but rather depends on the microscopic details of the model under consideration. Numerical investigations of the nearest-neighbor model are carried out {\it via} zero-temperature series expansions about Ising and dimer Hamiltonians. The results are entirely consistent with a vanishing critical interchain coupling ratio $R_c$; if $R_c$ is finite, it is unlikely to substantially exceed 0.02. 
Ian Affleck, Martin P. Gelfand, and Rajiv R. P. Singh