1. CEMS, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
2. Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
3. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai 319-1195, Japan
4. Kavli Institute for Theoretical Sciences, University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China