1. Institute for Nonlinear Science, University of California, San Diego, La Jolla, California 92093-0402
2. Max-Planck Institut für Festkörperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany
3. Department of Chemistry and Biochemistry, 0340, and Institute for Nonlinear Science, University of California, San Diego, La Jolla, California 92093-0340