1. Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Naka, Ibaraki 319-1195
2. Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043
3. Low Temperature Center, Osaka University, Toyonaka, Osaka 560-0043
4. Department of Physics, Tokyo Metropolitan University, Minami-Ohsawa 1-1, Hachioji, Tokyo 192-0397
5. Graduate School of Science, Hokkaido University, Sapporo 060-0810
6. Department of Materials Science, Japan Atomic Energy Research Institute, Tokai, Naka, Ibaraki 319-1195
7. The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047
8. Graduate School of Science, Tohoku University, Sendai 980-8578