Spin Correlations in the Paramagnetic Phase and Ring Exchange inLa2CuO4

Spin correlations in the paramagnetic phase of ${\mathrm{La}}_2{\mathrm{CuO}}_4$ have been studied using polarized neutron scattering, with two important results. First, the temperature dependence of the characteristic energy scale of the fluctuations and the amplitude of the neutron structure factor are shown to be in quantitative agreement with the predictions of the quantum nonlinear sigma model. Second, a comparison of a high-temperature series expansion of the equal-time spin correlations with the diffuse neutron intensity provides definitive experimental evidence for ring exchange.