1. Faculty of Education, Ehime University, Matsuyama 790-8577, Japan
2. Faculty of Education, Kumamoto University, Kumamoto 860-8555, Japan
3. Department of Physics, University of Science, Unjong-District, Pyongyang, D. P. R. Korea
4. Institute for Solid State Physics, Darmstadt University of Technology, D-64289 Darmstadt, Germany