1. School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, United States
2. Department of Physics, The Ohio State University, Columbus, Ohio 43210, United States
3. Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242, United States
4. Department of Applied Physics, Eindhoven University of Technology, Eindhoven 5612 AZ, The Netherlands
5. Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, United States