1. Department of Engineering Science, University of Electro-Communications, Chofu, Tokyo 182-8585, Japan
2. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
3. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
4. Department of Physics, Kyushu University, Fukuoka 812-8581, Japan