High-energy spin excitations in the insulating phases of high-Tc superconducting cuprates and La2NiO4

Abstract
High-energy spin excitations up to above 8000 cm1 were measured by polarized Raman scattering in the insulating phases of S=1/2 copper oxides, La2 CuO4, YBa2 Cu3 O6.2, Bi2 Sr2 Ca0.5 Y0.5 Cu2 O8+y, Nd2 CuO4, and Pr2 CuO4, and an S=1 nickel oxide La2 NiO4. 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.