This study is supported jointly by the funds for Supporting Project for State Science and Technology (No.2006BAB01B09);the Natural Science Foundation of Yunnan Province (No.99D0003G);the Collaboration Program sponsored by the colleges and universities of Yunnan Province (No. 2002UBBEA05B004);the funds for Program for New Century Excellent Talents in University (No.NCET- 04- 917).