1. Graduate School of Engineering and Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
2. Faculty of Science, Shinshu University, Matsumoto, Nagano 390-8621, Japan
3. Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313, Japan
4. Low Temperature Center, Osaka University, Toyonaka, Osaka 560-0043, Japan
5. Graduate School of Science, Kobe University, Kobe 657-8501, Japan
6. Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan