**Author details**

J.C. Fan1 , S.L. Chang2 and Z. Xie1

1 College of Physics and Microelectronics Science, Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, Hunan University, Changsha 410082, People's Republic of China

2 Center of Materials Science, College of Science, National University of Defense Technolo‐ gy, Changsha, 410073, People's Republic of China
