NEUTRON-SCATTERING STUDY OF THE 2-DIMENSIONAL SPIN S=1 2 SQUARE-LATTICE HEISENBERG-ANTIFERROMAGNET SR2CUO2CL2/

We have carried out a neutron scattering investigation of the static s tructure factor S(q(2D)) (q(2D) is the in-plane wave vector) in the tw o-dimensional spin S = 1/2 square-lattice Heisenberg antiferromagnet S r2CuO2Cl2. For the spin correlation length xi we find quantitative agr eement with Monte Carlo results over a wide range of temperature. The combined Sr2CuO2Cl2-Monte Carlo data, which cover the length scale fro m similar to 1 to 200 lattice constants, are predicted without adjusta ble parameters by renormalized classical theory for the quantum nonlin ear sigma model. For the structure factor peak S(0), on the other hand , we find S(0)similar to xi(2) for the reduced temperature range 0.16 < T/2 pi rho(S) < 0.36, whereas current theories predict that at low t emperatures S(0)similar to T-2 xi(2). This discrepancy has important i mplications for the interpretation of many derivative quantities such as NMR relaxation rates. In the ordered phase, we have measured the te mperature dependence of the out-of-plane spin-wave gap. Its low-temper ature value of 5.0 meV corresponds to an XY anisotropy J(XY)/J=1.4 x 1 0(-4). From measurements of the sublattice mangetization we obtain bet a=0.22+/-0.01 for the order parameter exponent. This may either reflec t tricriticality as in La2CuO4, or it may indicate finite-size two-dim ensional XY behavior as suggested by Bramwell and Holdsworth. As in th e S = 1 system K2NiF4, the gap energy in Sr2CuO2Cl2 scales linearly wi th the order parameter up to the Neel temperature. We also reanalyze s tatic structure factor data for K2NiF4 using the exact low temperature result for the correlation length of Hasenfratz and Niedermayer and i ncluding the Ising anisotropy explicitly. Excellent agreement between experiment and theory is obtained for the correlation length, albeit w ith the spin-stiffness rho(s) reduced by similar to 20% from the spin- wave value. As in Sr2CuO2Cl2 we find that S(0)similar to xi(2) for the reduced temperature range 0.22 < T/2 pi rho(S) < 0.47.