Acknowledgments
This research was supported by the National Basic Research Program of China (Program 973) (No. 2009cb724606) and the Natural Science Fund for Creative Research Groups in China (No.
50621403), the Fundamental Research Fund of the central Uni- versities of China (No. CDJZR10240017), and the Natural Science Foundation Project of CQ CSTC (No. 2010BB6044), which are all greatly appreciated