1. Department of Pure and Applied Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902
2. Faculty of Engineering, Takushoku University, 815-1 Tatemachi, Hachioji, Tokyo 193-0985
3. Service de Physique Théorique, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France