1. Department of Electrical Engineering, Columbia University, New York, New York 10027, United
States
2. School of Science, Northwestern Polytechnical University, Xi’an
710072, China
3. Department
of Physics, Columbia University, New York,
New York 10027, United
States
4. Department of Mechanical
Engineering, Columbia University, New York,
New York 10027, United
States
5. Department of Physics, Humboldt-Universität zu Berlin, Newtonstrasse
15, 12489 Berlin, Germany
6. Department
of Applied Physics and Applied
Mathematics, Columbia University, New York,
New York 10027, United States