1. Graduate School of Science, Osaka City University, Osaka 558-8585, Japan
2. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
3. National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32306-4005, U.S.A.
4. Laboratoire de Physique des Solides, Université Paris-Sud, 91405 Orsay, France
5. H. C. Oersted Institute, Universitetsparken 5, DK 2100 Copenhagen, Denmark
6. Inststitute for Molecular Science, Okazaki, Aichi 444-8585, Japan
7. Department of Physics, Okayama University, Okayama 700-8530, Japan
8. RIKEN, Wako, Saitama 351-0198, Japan