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 S-1/3 smaller than for conventional spin waves. The relevance of these results to the experiments on the stacked kagomé antiferromagnet SrCr8-xGa4+xO19 is discussed.