1. Department of Physics, Keio University, Yokohama 223
2. Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106
3. Booster Synchrotron Utilization Facility, National Laboratory for High Energy Physics, Tsukuba 305
4. Department of Materials Science and Engineering, Japan Atomic Energy Research Institute, Tokai 319-11