1. Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
2. Research Institute for Electronic Science, Hokkaido University, N21W10, Sapporo 001-0021, Japan
3. Hitachi Research Laboratory, Hitachi Limited, 7-1-1 Omika, Hitachi, Ibaraki 319-1292, Japan