Abstract
The spin excitations in La
2CuO
4, a two-dimensional spin-
1
2
Heisenberg antiferromagnet and parent compound of high-
T
c superconductors, have been measured throughout the Brillouin zone using high-resolution inelastic neutron scattering. The observed dispersion relations show evidence for substantial second- or third-nearest-neighbour exchange couplings in addition to the strong first nearest-neighbour antiferromagnetic exchange.