1. Contribution from the School of Chemistry, Physics and Environmental Science, University of Sussex, Brighton BN1 9QJ, U.K., Max-Planck-Institut für Metallforschung, Seestrasse 92, 70174 Stuttgart, Germany, Instituto de Física, UNAM, AP 1-1010, 76000, Querétaro, México, and Materials Research Laboratory, University of California, Santa Barbara, California 93106