1. Department of Physics, Waseda University, Shinjuku, Tokyo 169-8555, Japan
2. Department of Basic Science, University of Tokyo, Meguro, Tokyo 153-8902, Japan
3. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.
4. Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, Shinjuku, Tokyo 169-0051, Japan