  Spin correlations in the paramagnetic phase of lanthanum cuprate 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 non-linear sigma
model. Secondly, 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.
