Ring exchange in lanthanum cuprate

We have measured the diffuse scattering of polarized neutrons in the paramagnetic phase of La2CuO4 using the IN20 spectrometer at the Institut Laue-Langevin. The dynamical response is shown to be in full agreement with the predictions of the quantum nonlinear sigma model. The staggered susceptibilty has been placed on an absolute scale to facilitate the comparison of our data with theoretical models. New calculations of the high-temperature series expansion of the spin correlations in the paramagnetic phase provide compelling, quantitative evidence for the existence of four-particle ring exchange