1. Department of Physics, Graduate School of Science, Kobe University, Kobe 657-8501, Japan
2. Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
3. Center for Low Temperature Science, Tohoku University, Sendai 980-8578, Japan
4. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
5. Department of Physics, Graduate School of Science, University of Ryukyus, Nishihara, Okinawa 903-0213, Japan