1. Graduate School of Material Science, University of Hyogo, Koto, Kamigori, Ako-gun, Hyogo 678-1297, Japan
2. Department of Physics and Electronics, Graduate School of Engineering, Osaka Prefecture University, Sakai 599-8531, Japan
3. Department of Applied Physics, Graduate School of Engineering, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan