1. Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8561, Japan
2. Japan Aerospace Exploration Agency, Tsukuba, Ibaraki 305-8505, Japan
3. Department of Materials Science and Engineering, University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
4. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan