1. Department of Physics, School of Science and Engineering, Waseda University, Tokyo 169-8555
2. Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573
3. Department of Physics, Kyushu University, Fukuoka 812-8581
4. Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195
5. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581
6. Institute for Materials Research, Tohoku University, Sendai 980-8577