[1] Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
[2] Forschungszentrum Karlsruhe, Institut für Festkörperphysik, P.O. Box 3640, 76021 Karlsruhe, Germany
[3] Fakultät für Physik, Universität Karlsruhe, 76131 Karlsruhe, Germany
[4] CREST, Japan Science and Technology Corporation and National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan