1. Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University, Ithaca, New York 14853, USA
2. School of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14853, USA
3. Physics Department, University of Notre Dame, Notre Dame, Indiana 46556, USA
4. Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA
5. Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA