1. Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, USA.
2. Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China.
3. Department of Mechanical Engineering, Columbia University, New York, NY 10027, USA.
4. Department of Physics and Astronomy, College of Arts and Sciences, Howard University, Washington, DC 20059, USA.