1. Department of Physics, College of Science and Technology, Nihon University, 1-8 Kanda Surugadai, Chiyoda-ku, Tokyo 101-8308
2. Nanoelectronics Research Institute, AIST, Tsukuba, Ibaraki 305-8568
3. JSPS, Chiyoda-ku, Tokyo 102-8471
4. DuPont Co., Central Research and Development, E356/209, Wilmington, DE 10998-0356, U.S.A.