1. Laboratoire de Physique de la Matière Condensée et Nanostructures, Université Lyon 1, CNRS, UMR 5586, Domaine Scientifique de la Doua, F-69622 Villeurbanne cedex, France, and Richard E. Smalley Institute for Nanoscale Science and Technology and Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas 77005-1892