HIGH-ENERGY SPIN EXCITATIONS IN THE INSULATING PHASES OF HIGH-TC SUPERCONDUCTING CUPRATES AND LA2NIO4

High-energy spin excitations up to above 8000 cm-1 were measured by polarized Raman scattering in the insulating phases of S=1/2 copper oxides, La2CuO4, YBa2Cu3O6.2, Bi2Sr2Ca0.5Y0.5Cu2O8+y, Nd2CuO4, and Pr2CuO4, and an S=1 nickel oxide La2NiO4. All the copper oxides commonly show anomalous spectra, especially the secondary scattering peaks near 4J (J is the two-spin superexchange energy). Those anomalies are caused by the large four-spin cyclic exchange interactions. The present experiments demonstrated that the four-spin exchanges show distinctive characteristics in cuprates as compared to other materials. © 1990 The American Physical Society.