Spin Waves and Electronic Interactions in La2CuO4

The magnetic excitations of the square-lattice spin-1/2 antiferromagnet and high- T_c parent compound La₂CuO₄ are determined using high-resolution inelastic neutron scattering. Sharp spin waves with absolute intensities in agreement with theory including quantum corrections are found throughout the Brillouin zone. The observed dispersion relation shows evidence for substantial interactions beyond the nearest-neighbor Heisenberg term which can be understood in terms of a cyclic or ring exchange due to the strong hybridization path around the Cu₄O₄ square plaquettes.