1. Graduate School of Engineering and Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
2. Low Temperature Center, Osaka University, Toyonaka, Osaka 560-0043, Japan
3. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
4. Graduate School of Science, Kobe University, Kobe 657-8501, Japan
5. Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan