基金Acknowledgments The authors would like to thank the editors and the anonymous referees for their helpful suggestions and comments which led to the improvement of our original manuscript. This work is supported by the National Natural Science Foundation of China (11372294 and 11261017), Scientific Research Fund of Sichuan Provincial Education Department (14ZB0115 and 15ZB0113), Zhejiang Provincial Natural Science Foundation (LQ13A010023) and Doctorial Research Fund of Southwest University of Science and Technology (15zx7138).