1. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
2. Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
3. Low Temperature Center, Osaka University, Toyonaka, Osaka 560-0043, Japan
4. KYOKUGEN, Osaka University, Toyonaka, Osaka 560-8531, Japan
5. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan