1. Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213
2. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581
3. Faculty of Engineering, University of the Ryukyus, Nishihara, Okinawa 903-0213
4. A. F. Ioffe Physico-Technical Institute, Russian Academy of Sciences, Sankt-Petersburg 194021, Russia