1. Center for Emergent Matter Science, RIKEN, 2-1 Hirosawa, Wako 351-0198, Japan
2. Department of Computer Science and Electronics, Kyushu Institute of Technology, 680-4 Kawazu, Iizuka 820-8502, Japan
3. Research and Development Group, Hitachi, Ltd., 1-280 Higashi-koigakubo, Kokubunji-shi, Tokyo 185-8601, Japan
4. Institute for Solid State Physics, University of Tokyo, Kashiwa 277-858, Japan