1. Nano Electronics Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japan, SORST, Japan Science and Technology Agency, c/o NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japan, Laboratoire de Physique des Solides, CNRS UMR 8502, 91405 Orsay, France, Department of Chemistry, Meijo University, 1-501 Shiogamaguchi, Nagoya 468-8502, Japan