1. Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8561, Japan.
2. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan.
3. Department of Physics, Nippon Institute of Technology, Miyashiro, Saitama 345-8501, Japan.
4. Institute for Materials Research, Tohoku University, Aoba-ku, Sendai 980-8577, Japan.
5. Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
6. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.