MAGNETIC-ORDERING AND FLUCTUATIONS IN THE S = 1/2 SQUARE HEISENBERG-ANTIFERROMAGNET CU(DCO2)2.4D2O

Elastic and quasielastic neutron scattering measurements have been performed on the S = 1/2 square Heisenberg antiferromagnet copper (II) deuteroformate tetradeuterate, Cu(DCO2)2 . 4D2O. The sublattice magnetization was measured below T(N) = 16.54(5) K, and the critical exponent-beta was found to cross over from 0.23(1) to 0.32(2) at a reduced temperature epsilon = (T(N) - T)/T(N) of 0.06 as T(N) was approached from below. The temperature dependence of the inverse correlation length of the two-dimensional antiferromagnetic correlations and of the sublattice susceptibility above T(N) was consistent with the predictions of the quantum nonlinear sigma model of Chakravarty and co-workers for a spin-wave stiffness of 13.4(4) K.