ORDER FROM DISORDER IN A KAGOME ANTIFERROMAGNET

A Heisenberg antiferromagnet on a kagome lattice is highly degenerate in the classical limit. I show that quantum fluctuations lift the degeneracy and yield 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 kagome antiferromagnet SrCr8-xGa4+xO19 is discussed.