1. Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043
2. KYOKUGEN, Osaka University, Toyonaka, Osaka 560-0043
3. ISSP, University of Tokyo, Kashiwa, Chiba 277-8581
4. Low Temperature Center, Osaka University, Toyonaka, Osaka 560-0043
5. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195
6. Department of Physics, College of Humanities and Sciences, Nihon University, Setagaya, Tokyo 156-8550