1. Department of Advanced Materials Science, The University of Tokyo, Kashiwa, Chiba 277-8561, Japan
2. The Institute for Solid State Physics (ISSP), The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
3. Research Institute for Advanced Electronics and Photonics, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
4. Material Innovation Research Center (MIRC), The University of Tokyo, Kashiwa, Chiba 277-8561, Japan
5. Office of University Professor, The University of Tokyo, Kashiwa, Chiba 277-8568, Japan
6. Department of Physics, Nagoya University, Nagoya 464-8602, Japan