1. Department of Physics, Teikyo University, 1-1 Toyosatodai, Utsunomiya 320
2. Department of Physics, Ochanomizu University, Bunkyo-ku, Tokyo 112
3. Institute for Solid State Physics, The University of Tokyo, Minato-ku, Tokyo 106
4. Department of Physics, Faculty of Science, Kyushu University, Higashi-ku, Fukuoka 812-81