1. National Laboratory of Solid State Microstructures School of Physics Collaborative Innovation Center of Advanced Microstructures Nanjing University Nanjing 210093 P. R. China
2. Institute of Interdisciplinary Physical Sciences School of Science Nanjing University of Science and Technology Nanjing 210094 P. R. China
3. Science, Mathematics and Technology Singapore University of Technology and Design Singapore 487372 Singapore
4. Physics Department Harvey Mudd College Claremont CA 91711 USA