1. National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan; School of Science, The University of Tokyo, 7−3−1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan; and Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany