This research was partly supported by the National Natural Science Foundation of China (nos 11571216 and 11401184), the Hunan Province Natural Science Foundation of China (no. 2015JJ3025), China's Fifty-ninth Batch of Postdoctoral Foundation (no. 2016M590492), the Jiangsu Province Postdoctoral Foundation of China (no. 7131710516), the Science and Technology Plan Project of Hunan Province (no. 2016TP1020) and the Construct Program of the Key Discipline in Hunan Province. The second author is on leave from IIT Madras.