1. Nanomaterials Laboratory, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan, PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho-Kawaguchi 331-0012, Japan, and I.N.F.M. and Dipartimento di Scienza dei Materiali, Universitá di Milano Bicocca, Via Cozzi, I-20125 Milano, Italy