基金This work was supported by the following funds: National Natural Science Foundation of China (No.30670951) Guangdong Provincial+5 种基金 Natural Science Foundation (No.06021322) Fund of Guangzhou Municipal Scientific Problem-Solving Program (No. 2003 Z 3-E0381) Fund of Guangdong Provincial Scientific Problem-Solving Program (No.2005 B31211002) Guangdong Provincial Government and Ministry of Education Project com- bining project initiation, study and research (No.2009B090300277).