1. School of Physics, Nanjing University, Nanjing 210093, People's Republic of China
2. Kuang Yaming Honors School, Nanjing University, Nanjing 210093, People's Republic of China and Department of Physics, Duke University, North Carolina 27710
3. Kuang Yaming Honors School, Nanjing University, Nanjing 210093, People's Republic of China and Department of Electrical and Computer Engineering, The University of Texas at Austin, Texas 78712