1. Nanosimulation Research Group, Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 2-1, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568
2. Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656