1. Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397
2. Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501
3. NSL, Institute for Solid State Physics, University of Tokyo, Tokai, Ibaraki 319-1106
4. NCNR, National Institute of Standards and Technology, Gaithersburg, MD 20899-6102, U.S.A.
5. Department of Electronics, Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550
6. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581