We thank S.R.Bickham, G.V.Chester, R.H.Silsbee and M.P.Teter for helpful discussions. This work is supported in part by NSF-DMR-9631298, ARO-DAAH04-96-1-0029, NATO-CRG 960636 and the MRL central facilities. Some of this research was conducted using the resources of the Cornell Theory Center, which receives major funding from the National Science Foundation and New York State, with additional support from other members of the center's Corporate Partnership Program.
Last modified: April 3, 1997