基金Acknowledgments: This work was funded by the New Century Talent Support Program (No. NCET-04-0746) and the Region Technology Development Program of Chinese Ministry of Education (No. 02095), and the Natural Science Foundation of Hubei province (No. 2002AB094), the Youth Talent Foundation of Hubei province (No. 2003AB014) and the Educational Office Key Research Program of Hubei Province of China (No. Z200627002).