基金We thank journal editors and anonymous reviewers for their constructive comments and careful corrections that lead to significant improvement of the manuscript. Also thanks are given to Dr. Yan Zhan, Associate Prof. Qiugen Li, Associate Prof. En Meng, Prof. Fuping Pei, Dr. Meng Wang, Dr. Jingwei Liu and Dr. Lu Zhang for their discussion and drawing figures. This work was financially supported by the National Key Basic Research Program of China （Grant No. 2012CB416603）, the National Natural Science Foundation of China （Grant No. 90914001）, the Opening Foundation of the Peking University （Grant No. 0000010541） and Undergraduates Innovating Experimentation Project of Jilin University （Grant No. 2010C61164）.