1. Department of Physics, Faculty of Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162-8601
2. Hahn-Meitner-Institute, Glienicker Straße 100, D-14109 Berlin, Germany
3. National Research Institute for Metals, Tsukuba-shi, Ibaraki 305-0047
4. RIKEN (The Institute of Physical and Chemical Research) Wako, Saitama 351-0198