1. Graduate School of Science and Engineering, University of Toyama, Toyama 930-8555, Japan
2. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
3. Low Temperature Center, Osaka University, Toyonaka, Osaka 560-0043, Japan
4. The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan