基金Acknowledgements： This work is supported by the National Natural Science Foundation of China （No. 60473012）, the Natural Science Foundation of Jiangsu Province of China （No. BK2005047, BK2004052 and BK2005046）, the Tenth-Five High Technology Key Project of Jiangsu Province of China （No. BG2004034）, and the Natural Science Foundation of yangzhou University （No. KK0413161）.
