[1] Lab. for Electron. Intelligent Syst., Res. Inst. of Elec. Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-77, Japan
[2] Shanghai Inst. of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China
[3] Inst. for Microstructural Sciences, National Research Council, Ottawa, Ont. K1A OR6, Canada