1. Advanced Science Institute, RIKEN, Hirosawa 2-1, Wako, Saitama, 351-0198, Japan
2. Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan
3. Department of Physics, Kobe University, Kobe, 657-8501, Japan
4. Department of Advanced Materials, University of Tokyo, Kashiwa, 277-8561, Japan