1. Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Naka, Ibaraki 319-1195
2. Department of Physics, Tohoku University, Sendai 980-8578
3. Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043
4. Research Center for Materials Science at Extreme Conditions, Osaka University, Toyonaka, Osaka 560-8531