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 La2Cu1−z(Zn,Mg)zO4 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 (SL-HAF) in the quantum-spin limit S = 1/2. Carefully oxygen-reduced samples exhibit Néel order up to the percolation threshold for site dilution, zp ≈ 40.7%. For z> 10%, the material exhibits a low-temperature tetragonal (LTT) structural phase, with a transition temperature that increases linearly with doping. Above z ≈ 25%, Néel 1 order occurs in the LTT phase. Up to at least z = 35%, the Néel temperature TN(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