Spin correlations in the paramagnetic phase and ring exchange in La2CuO4
We have developed a new way of determining magnetic exchange constants using spin correlations in the paramagnetic phase by single-crystal scattering of polarised neutrons. This work has relied on the development of three-dimensional polarization-analysis techniques to isolate the purely magnetic scattering, and close collaboration with theorists to interpret the results. One notable success is the discovery of “ring exchange” in La2CuO4, and this is the first time that multi-particle exchange has been observed in an electronic magnetic material. Cuprate high-temperature superconductors are the most widely studied materials in history, but until now we have not even been able to write down the spin Hamiltonian, the first and most important ingredient in any theoretical model. Furthermore, the temperature dependence of the characteristic energy scale and the amplitude of the neutron structure factor are shown to be in quantitative agreement with the predictions of the quantum nonlinear sigma model.