1. Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195
2. Department of Basic Science, University of Tokyo, Meguro-ku, Tokyo 153-8902
3. Institute for Material Research, Tohoku University, Oarai, Ibaraki 311-1313
4. Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043