1. Department of Physics and Mathematics, Aoyama Gakuin University, Sagamihara, Kanagawa 229-8558
2. Institute of Materials Structure Science, KEK, Tsukuba, Ibaraki 305-0801
3. Department of Physics, Tohoku University, Sendai 980-8578
4. Advanced Science Research Center, JAERI, Tokai, Ibaraki 319-1195
5. Institute of Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581