1. Department of Applied Physics and Physico-Informatics, Keio University, Yokohama 223-8522, Japan
2. Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552, Japan
3. PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan