1. Institute of Physics, University of Tsukuba, Tennoudai 1-1-1, Tsukuba 305, Japan
2. Institute for Solid State Physics, University of Tokyo, Roppongi 7-22-1, Minato-ku, Tokyo 106, Japan
3. Department of Physics, Faculty of Science, Hokkaido University, Kita 10 Nishi 8, Kita-ku, Sapporo 060, Japan