1. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195
2. Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313
3. Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043
4. Department of Physics, Faculty of Science, Kobe University, Kobe 657-8501