1. Department of Physics, Saitama Medical University, Moroyama, Saitama 350-0495, Japan
2. The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
3. Faculty of Education, Chiba University, Chiba 273-8522, Japan
4. Department of Physics, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan