1. Thomas
J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, United States
2. School
of Electrical Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea
3. Kavli
Nanoscience Institute, California Institute of Technology, Pasadena, California 91125, United States
4. Department
of Physics, University of Wisconsin—Madison, Madison, Wisconsin 53711, United States