1. Department of Physics, The Hong Kong University of Science and Technology 1 , Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China
2. State Key Laboratory for Strength and Vibration of Mechanical Structure, School of Aerospace Engineering, Xi’an Jiaotong University 2 , Xi’an 710049, China
3. 3 William Mong Institute of Nano Science and Technology, Hong Kong University of Science and Technology, Hong Kong, China