1. Department of Material Science and Engineering, Cornell University, Ithaca, New York 14853, USA
2. School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA
3. Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University, Ithaca, New York 14853, USA
4. Department of Electrical Engineering, IIT Kanpur, Kanpur 208016, India
5. School of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14853, USA
6. Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, Tokyo 113-8656, Japan
7. Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
8. Leibniz Institute for Crystal Growth, D-12489 Berlin, Germany
9. Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA