1. Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University 1 , Ithaca, New York 14853, USA
2. School of Applied and Engineering Physics, Cornell University 2 , Ithaca, New York 14853, USA
3. Kavli Institute at Cornell for Nanoscale Science, Cornell University 3 , Ithaca, New York 14853, USA
4. Department of Materials Science and Engineering, Cornell University 4 , Ithaca, New York 14853, USA
5. Leibniz-Institut für Kristallzüchtung 5 , Max-Born-Straße 2, 12489 Berlin, Germany
6. Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB 6 , Bellaterra 08193, Spain