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Flat dispersion in a triangular antiferromagnet


I present the results for the excitation spectrum of a S = 1/2 2D triangular quantum
antiferromagnet, which we obtained using 1/S expansion. I show that 1/S corrections to
a classical (S = 1) result are unexpectedly large throughout the Brillouin zone. Quite
unexpectedly, the resulting spin wave dispersion is almost flat in a substantial portion of
the Brillouin zone. The results are in quantitative agreement with recent series expansion
studies by R.R.P. Singh et al. I discuss how the flat dispersion affects Raman and other