1. Institute for Material Research, Tohoku University, Sendai 980-8577, Japan
2. School of Science, the University of Tokyo, Bunkyo, Tokyo 113-8654, Japan
3. Institute for Solid State Physics, the University of Tokyo, Kashiwa, Chiba 277-8581, Japan
4. School of Medicine, Nihon University, Itabashi, Tokyo 173-8610, Japan
5. Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
6. National High Magnetic Field Laboratory, Tallahassee, FL 32310-3706, U.S.A.
7. Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan