1. RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
2. Department of Advanced Materials Science, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan
3. Division of Materials Science, Korea Basic Science Institute, Daejeon 305-333, Republic of Korea