1. Department of Materials Science and Engineering, University of Wisconsin─Madison, Madison, Wisconsin 53706, United States
2. Department of Physics, University of Wisconsin─Madison, Madison, Wisconsin 53706, United States
3. Department of Electrical and Computer Engineering, University of Wisconsin─Madison, Madison, Wisconsin 53706, United States
4. Faculty of Science, University of Hong Kong, Hong Kong, China
5. Faculty of Engineering, University of Hong Kong, Hong Kong, China