1. Department of Materials 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. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA
4. Department of Physics, University of California, Berkeley, California 94720, USA
5. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
6. Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA
7. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA
8. Leibniz-Institut fur Kristallzüchtung, Max-Born-Str. 2, 12489 Berlin, Germany