1. Department of Applied Physics, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
2. Key Laboratory of Instrumentation Science and Dynamic Measurement, School of Instrument and Electronics, North University of China, Taiyuan, Shanxi 030051, P. R. China
3. Low-Dimensional and Surface Physics, Department of Physics, Renmin University of China, LiGongLou 714, Beijing 100872, P. R. China