1. Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043
2. Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195
3. Low Temperature Center, Osaka University, Toyonaka, Osaka 560-0043
4. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581