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