1. Nano Electronics Research Laboratories and Bio-IT Center, Central Research Laboratories, NEC Corporation, 34, Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan, and Riken, Next-Generation Supercomputer R&D Center, sixth Fl., Meiji Seimei Kan, 2-1-1 Marunouchi, Chiyoda-ku, Tokyo 100-0005