1. Research Center for Extreme Materials, Osaka University, Toyonaka, Osaka 560
2. Department of Physics, Faculty of Science, Osaka University, Toyonaka, Osaka 560
3. Department of Physics, Faculty of Science, Tohoku University, Sendai 980
4. Department of Physics, Miyagi University of Education, Sendai 980
5. Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokaimura, Naka-gun, Ibaraki 319-11