1. Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8561, Japan
2. Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
3. Department of Applied Physics, University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
4. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan