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