1. Department of Physics, Faculty of Science, Kyushu University, Higashi, Fukuoka 812-81
2. Department of Electronics, Tohoku Institute of Technology, Taihaku, Sendai 982
3. Department of Applied Physics, Faculty of Engineering, Tohoku University, Aoba, Sendai 980-27
4. Department of Physics, School of Science and Engineering, Waseda University, Shinjuku, Tokyo 169
5. Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 315