1. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
2. National Institute for Materials Science, Tsukuba, Ibaraki 305-0003, Japan
3. Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan
4. Max Planck Institute for Chemical Physics of Solids, Nothnitzer Str. 40, 01187 Dresden, Germany
5. Department of Physics, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
6. Department of Basic Science, The University of Tokyo, Meguro, Tokyo 153-8902, Japan
7. Condensed Matter Theory Laboratory, RIKEN, Wako, Saitama 351-0198, Japan
8. Research Organization of Science and Technology, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan