SPIN DYNAMICS IN ND2CUO4 AND PR2CUO4

The low-energy part of the copper spin-wave excitation spectrum in the undoped antiferromagnets Nd2CuO4 and Pr2CuO4 has been studied by inelastic neutron scattering. In Nd2CuO4, magnetic scattering observed around the (1/2 1/2 Q) rod exhibits pronounced energy gaps for both the ''in-plane'' and ''out-of-plane'' spin fluctuations over the whole Brillouin zone. The two gaps appear to be dispersionless along the tetragonal axis direction and remarkably increase when temperature decreases. In Pr2CuO4, the spin-wave gaps are also observed with smaller energies at low temperatures.