supported by funds from the Natural Science Fundation of China (No.40874034,40537033,);Program for New Century Excellent Talents in University (NCET-05-0553);the graduate innovation of USTC (No.KD2007059);funds of Heifei University Technology (No. 2009HGXJ002);COE-21 Collaborative Research Program at ISEI, Misasa, Japan