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, Faculty of Science, Shinshu University, Matsumoto, Nagano 390-8621
4. Low Temperature Center, Osaka University, Toyonaka, Osaka 560-0043
5. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581