1. National Institute for Materials Science, Tsukuba, Ibaraki 305-0003, Japan
2. Department of Physics, Kyoto University, Kyoto 606-8502, Japan
3. Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8561, Japan
4. Institute of Solid State Physics (IFP), Karlsruhe Institute of Technology, D-76021 Karlsruhe, Germany