1. Faculty of Science, University of the Ryukyus, Okinawa 903-0213
2. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581
3. Muroran Institute of Technology, Muroran, Hokkaido 050-8585
4. Institute for Material Research, Tohoku University, Sendai 980-8577
5. Institute for Advanced Materials Research, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8530
6. A. F. Ioffe Physical-Technical Institute, St. Petersburg 194021, Russia