1. Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan, Research Center for Ultrahigh Voltage Electron Microscopy, Osaka University, Ibaraki, Osaka 567, Japan, and Nokia Research Center, Nokia-Japan Co., Ltd., 2-13-5, Nagata-cho, Chiyoda-ku, Tokyo 100-0014, Japan