基金This work was supported by the National Natural Science Foundation of China(0072058)863 Foundation of China(2003AA624010)+2 种基金the National Natural Science Foundation of Guangdong Province,China(021732,2003A 3050401)a strategic grant at City University of Hong Kong(7000650)E.B.G.J.acknowledges the Royal Society,U.K.and City University of Hong Kong for the award of the Kan Tong Po Visiting Professorship.