1. Graduate School of Science and Engineering, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
2. General Education, Okinawa Christian Junior College, Nishihara, Okinawa 903-0207, Japan
3. Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
4. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
5. A. F. Ioffe Physical-Technical Institute, Russian Academy of Sciences, Sankt-Petersburg 194021, Russia