1. Graduate School of Material Science, University of Hyogo, Kamigori, Hyogo 678-1297, Japan
2. Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 679-5198, Japan
3. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
4. Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan