1. Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Fukuoka 819-0395, Japan
2. Research and Development Group, Hitachi Ltd., Hatoyama 350-0395, Japan
3. The Ultramicroscopy Research Center, Kyushu University, Fukuoka 819-0395, Japan
4. Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan