  The magnetic excitations of the square-lattice spin-1/2 antiferromagnet and
high-Tc parent 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.

Strongly Correlated Electrons;Superconductivity