1. Research Group for Surface and Interface Science, Advanced Science Research Center, Japan Atomic Energy Agency, 2-4 Shirakata, Tokai, Ibaraki 319-1195, Japan
2. Faculty of Engineering, Hokkaido University, North 13, West 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan
3. Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan
4. Department of Applied Physics, Osaka University, Suita, Osaka 565-0871, Japan
5. Center for Atomic and Molecular Technologies, Osaka University, Suita, Osaka 565-0871, Japan