1. Department of Physics and Astronomy & Nebraska Center for Materials and Nanoscience University of Nebraska–Lincoln Lincoln NE 68588‐0299 USA
2. Department of Mechanical and Materials Engineering & Nebraska Center for Materials and Nanoscience University of Nebraska–Lincoln Lincoln NE 68588‐0526 USA
3. Department of Physics Columbia University New York NY 10027 USA
4. New York University Shanghai Shanghai 200122 China
5. NYU‐ECNU Institute of Physics New York University Shanghai Shanghai 200062 China
6. Department of Physics New York University New York NY 10002 USA