1/S EXPANSION FOR DYNAMIC STRUCTURE FACTORS IN A 2-DIMENSIONAL HEISENBERG-ANTIFERROMAGNET AT ZERO TEMPERATURE

We calculate the dynamical structure factors of a two-dimensional Heisenberg antiferromagnet at T = 0, to order 1/(2S)2, taking careful account of the umklapp processes. We show that, for the transverse spin part, the terms of order 1/(2S)2 give rise to a broad sideband peak of three-magnon excitations, whose intensity is small but not negligible. For the longitudinal spin part, the terms of order 1/(2S)2 modify noticeably the spectral shape of the continuum of two-magnon excitations as a function of frequency. These results could be observed in future neutron scattering experiments.