1/S-expansion study of spin waves in a two-dimensional Heisenberg antiferromagnet -: art. no. 014403

We study the effects of quantum fluctuations on excitation spectra in the two-dimensional Heisenberg antiferromagnet by means of the 1/S expansion. We calculate the spin-wave dispersion and the transverse dynamical structure factor up to the second order of 1/S in comparison with inelastic neutron scattering experiments. The spin-wave energy at momentum (pi,0) is found to be about 2% smaller than that at (pi/2,pi/2) due to the second-order correction. In addition, we study the dimensional crossover from two dimensions to one dimension by weakening exchange couplings in one direction. It is found that the second-order correction becomes large with approaching the quasi-one-dimensional situation and makes the spin-wave energy approach to the des Cloizeaux-Pearson boundary for S=1/2. The transverse dynamical structure factor is also calculated up to the second order of 1/S. It is shown that the intensity of the spin-wave peak is strongly reduced while the intensity of three-spin-wave continuum becomes large and exceeds that of the spin-wave peak in the quasi-one-dimensional situation.