1. Department of Physics, Faculty of Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162
2. Institute for Solid State Physics, University of Tokyo, Roppongi, Tokyo 106
3. Physics Division, Faculty of General Education, Ehime University, Bunkyo-cho, Matsuyama 790
4. School of Science and Engineering, Waseda University, Shinjuku-ku, Tokyo 169