1. Department of Advanced Materials, University of Tokyo, Kashiwa, Chiba 277-8561, Japan
2. Max-Planck-Institute for Solid State Research, Heisenbergstrasse 1, 70569 Stuttgart, Germany
3. RIKEN Advanced Science Institute, Wako, Saitama 351-0198, Japan
4. Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan