[1] Tokyo Univ Agr & Technol, Grad Sch Engn, Koganei, Tokyo 1848588, Japan
[2] Kyushu Univ, Res Lab High Voltage Elect Micriscopy, Fukuoka 812, Japan
[3] Kyushu Univ, Art Sci Technol Ctr Corp Res, Fukuoka 812, Japan
[4] AF Ioffe Phys Tech Inst, St Petersburg 194021, Russia