Spin waves and electronic interactions in La2CuO4

The magnetic excitations of the square-lattice spin-1/2 antiferromagnet and high-ir, parent compound La2CuO4 are determined using high-resolution inel astic neutron scattering. Sharp spin waves with absolute intensities in agr eement with theory including quantum corrections are found throughout the;B rillouin zone. The observed dispersion relation shows evidence for substant ial interactions beyond the nearest-neighbor Heisenberg term which can be u nderstood in terms of a cyclic or ring exchange due to the; strong hybridiz ation path around the CU4O4 square plaquettes.