1. Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA.
2. Hefei National Laboratory for Physical Science at Microscale and Department of Physics, University of Science and Technology of China, Hefei Anhui 230026, China.
3. Department of Physics, Tsinghua University, Beijing, China.
4. Department of Physics and Astronomy, University of California at Irvine, Irvine, CA 92697, USA.