1. Department of Physics, Niigata University, Niigata 950-218, Japan
2. Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan
3. Institute for Materials Research, Tohoku University, Ibaraki 311-1313, Japan
4. Department of Condensed Matter Physics and Material Sciences, Tata Institute of Fundamental Research, Mumbai 400-005, India
5. Japan Synchrotron Radiation Research Institute, Hyogo 679-5198, Japan
6. Graduate School of Engineering, Osaka Prefecture University, Osaka 599-8531, Japan