Y.C. thanks the financial support by the Young Foundation of Science and Technology, Department of the Hunan province (grant no. 2015JJ3108), Scientific Research Fund of Hunan Provincial Education Department (grant no. 14B152 and 14C1000), Project of Research Study and Creativity Experiment Plan for College Students of the Hunan province (Hunan Provincial Education Department Bulletin grant no. [2016]283). Z.W. acknowledges financial support from the National Natural Science Foundation of China (grant no. 11332013)