[1] Department of Physics, University of Michigan, Ann Arbor,MI,48109, United States
[2] Department of Physics, College of Physical Science and Technology, Xiamen University, Xiamen,361005, China
[3] Department of Physics, Cornell University, Ithaca,NY,14853, United States
[4] School of Applied and Engineering Physics, Cornell University, Ithaca,NY,14853, United States
[5] Kavli Institute, Cornell for Nanoscale Science, Ithaca,NY,14853-2501, United States