1. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
2. Department of Applied Physics, University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
3. H. H. Wills Physics Laboratory, University of Bristol, Tyndall Av., BS8 1TL, U.K.
4. Research Institute for Solid State Physics and Optics, H-1525 Budapest, P.O.B. 49, Hungary