1. Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan
2. Institute for Solid State Physics, the University of Tokyo, Kashiwa, Chiba 277-8581, Japan
3. Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan
4. Dresden High Magnetic Field Laboratory, Helmholtz-Zentrum Dresden-Rossendorf, D-01314 Dresden, Germany