1. Department of Applied Physics, Waseda University, Shinjuku, Tokyo 169-8555, Japan
2. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
3. Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
4. Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany
5. Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan
6. Department of Quantum Matter, Hiroshima University, Higashihiroshima, Hiroshima 739-8530, Japan
7. Office of University Professor, University of Tokyo, Kashiwa, Chiba 277-8581, Japan