1. Advanced Technology Research Laboratories, Matsushita Electric Ind. Co., Ltd., 3‐1‐1 Yagumo‐Nakamachi, Moriguchi, Osaka 570‐8501, Japan
2. Department of Earth and Planetary Science, Tokyo Institute of Technology, 2‐12‐1 Ookayama, Meguro, Tokyo, 152‐8551, Japan