ELASTIC AND QUASI-ELASTIC NEUTRON-SCATTERING STUDY OF THE SPIN-1 2 SQUARE LATTICE ANTIFERROMAGNET SR2CUO2CL2/

Neutron scattering and magnetic susceptibility chi versus temperature T measurements are reported for a single crystal of the spin-1/2 squar e lattice Heisenberg antiferromagnet Sr2CuO2Cl2. Neutron Bragg diffrac tion measurements of the antiferromagnetic (1/2 1/2 0) peak showed tha t the Neel temperature is T-N = (256 +/- 6)K, whereas the chi(T) for m agnetic field applied parallel to the c-axis, chi(c)(T), showed a broa d minimum at T-c0 similar to 330 K, with no obvious feature at T-N. Th e two-dimensional (2D) quasielastic neutron scattering intensity, inte grated across the (1/2 1/2 l) 2D magnetic scattering rod, I(T), was fo und to increase approximately linearly with T from 10 to similar to 24 0 K, consistent with expectation from spin-wave theory. A broad maximu m in I(T) was found above T-N at T-c0 approximate to 300 K. Following B.J. Suh et al. [Phys. Rev. Lett. 75 (1995) 2212], we infer that this maximum and the corresponding broad minimum in chi(c)(T) at T-c0 > T-N reflect a crossover from Heisenberg to XY-like behavior upon cooling below T-c0 similar to 300 K. A simple model for I(T) supports this int erpretation.