1. Department of Physics, Faculty of Science, Tokyo Institute of Technology, Oh-okayama, Meguro-ku, Tokyo 152
2. Department of Physics, Faculty of Science and Technology, Sophia University, Kioi-cho, Chiyoda-ku, Tokyo 102
3. Department of Physics, Faculty of Science, Kobe University, Rokkodai, Kobe 657
4. Institute for Materials Research, Tohoku University, Sendai 980