Neutron Scattering, Magnetometry, and Quantum Monte Carlo Study of the   Randomly-Diluted Spin-1/2 Square-Lattice Heisenberg Antiferromagnet
  We have successfully grown sizable single crystals of La_2Cu_{1-z}(Zn,Mg)_zO_4 with up to nearly half of the magnetic Cu sites replaced by non-magnetic Zn and Mg. Neutron scattering, SQUID magnetometry, and complementary quantum Monte Carlo (QMC) simulations demonstrate that this material is an excellent model system for the study of site percolation of the square-lattice Heisenberg antiferromagnet (SLHAF) in the quantum-spin limit S=1/2. Carefully oxygen-reduced samples exhibit Neel order up to the percolation threshold for site dilution, z_p ~ 40.7%. Up to at least z = 35%, the Neel temperature T_N(z) of the experimental system corresponds to the temperature at which QMC indicates that the spin correlations for the nearest-neighbor S=1/2 SLHAF have grown to approximately 100 lattice constants. Neutron scattering measurements of the static structure factor in the paramagnetic regime allow the determination of the two-dimensional spin correlations, which are found to be in excellent quantitative agreement with QMC over a wide common temperature and doping range. Neutron scattering and QMC results for the temperature dependence of the static structure factor amplitude S(\pi,\pi) are in good agreement as well. The combined experimental and numerical data presented here provide valuable quantitative information for tests of theories of the randomly-diluted S=1/2 SLHAF. 
O. P. Vajk, M. Greven, P. K. Mang and J. W. Lynn