Order from disorder in a kagomé antiferromagnet

A Heisenberg antiferromagnet on a kagomé lattice is highly degenerate in the classical limit. I show that quantum fluctuations lift the degeneracy and yeild the low-energy branch of spin-wave excitations with a velocity which is a factor ${\mathit{S}}^{\mathrm{-}1/3}$ smaller than for conventional spin waves. The relevance of these results to the experiments on the stacked kagomé antiferromagnet ${\mathrm{SrCr}}_{8\mathrm{-}\mathit{x}}$ ${\mathrm{Ga}}_{4+\mathit{x}}$ ${\mathrm{O}}_{19}$ is discussed.