1. Department of Physics, Okayama University, Okayama 700-8530, Japan
2. Department of Materials Science, Shimane University, Matsue 690-8504, Japan
3. Department of Electrical and Electronic Engineering, Muroran Institute of Technology, Muroran, Hokkaido 050-8585, Japan
4. Department of Physics, Kobe University, Kobe 657-8501, Japan
5. JASRI/SPring-8, Sayo, Hyogo 679-5198, Japan
6. Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany