1. Graduate School of Engineering and Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
2. Department of Physics, Faculty of Science, Shinshu University, Matsumoto, Nagano 390-8621, Japan
3. Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313, Japan
4. Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan