Flat spin wave dispersion in a triangular antiferromagnet

The excitation spectrum of a S=1/2 two-dimensional triangular quantum antiferromagnet is studied using 1/S expansion. Due to the non-collinearity of the classical ground state significant and non-trivial corrections to the spin wave spectrum appear already in the first order in 1/S in contrast to the square lattice antiferromagnet. The resulting magnon dispersion is almost flat in a substantial portion of the Brillouin zone. Our results are in quantitative agreement with recent series expansion studies by Zheng, Fj\ae restad, Singh, McKenzie, and Coldea [PRL {\bf 96}, 057201 (2006) and cond-mat/0608008].