1. Department of Physics, Yokohama National University, 79-5 Tokiwadai, Yokohama 240-8501, Japan
2. Accelrys K. K., Kasumigaseki Tokyu Building 17F, 3-7-1 Kasumigaseki, Chiyoda-ku, Tokyo 100-0013, Japan
3. Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan