1. Department of Physics, University of Washington, Seattle, Washington 98195, USA
2. Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, USA
3. Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, New York 14853, USA
4. School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA
5. Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA