Flat spin-wave dispersion in a triangular antiferromagnet

The excitation spectrum of an $S=1∕2$ two-dimensional triangular quantum antiferromagnet is studied using $1∕S$ expansion. Due to the noncollinearity of the classical ground state significant and nontrivial 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 et al. [Phys. Rev. Lett. 96, 057201 (2006); cond-mat/0608008 (unpublished)].