1. Department of Physics, Tokyo Institute of Technology, Oh-okayama, Meguro-ku, Tokyo 152-8551
2. Service de Physique Théorique, CEA-Saclay, 91191 Gif-sur-Yvette Cédex, France
3. Institute for Solid State Physics, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8581