1. Fundamental Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan.
2. Department of Materials Science and Engineering, Northwestern University, 2225 North Campus Drive, Evanston, IL 60208, USA.
3. Japan Science and Technology Corporation, c/o Department of Physics, Meijo University, 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya 468-8502, Japan.