1. School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi 923-1292, Japan
2. OTOWA ELECTRIC CO., LTD., 5-6-20, Shioe Amagasaki 661-0976, Hyogo, Japan
3. School of Electronics and Computer Science, University of Southampton, Highfield, Southampton SO17 1BJ, U.K.