[1] Faculty of Integrated Arts and Sciences, Hiroshima University, Hiroshima 730, Japan
[2] Department of Materials Science, Faculty of Science, Hiroshima University, Higashi-Hiroshima 724, Japan
[3] College of General Education, Kyushu University, Fukuoka 820, Japan
[4] Department of Physics, Faculty of Science, Kyoto University, Kyoto 606, Japan