1/S expansion for dynamical structure factors in a two-dimensional Heisenberg antiferromagnet at zero temperature

The author calculates 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. It is shown 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.