1. Institute for Solid State Physics, the University of Tokyo, 5-1-5, Kashiwa-no-ha, Kashiwa, Chiba 277-8581, Japan
2. Physical Institute, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany
3. Department of Physics, National Taiwan Normal University, Taipei, 116, Taiwan
4. Department of Applied Physics, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan