1. Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043
2. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195
3. Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397
4. Low Temperature Center, Osaka University, Toyonaka, Osaka 560-0043
5. Department of Physics, Faculty of Science, Kobe University, Kobe 657-8501