1. Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany
2. Department of Physics, Cornell University, Ithaca, New York 14853, United States
3. Max Planck Institute for Microstructure Physics, 06120 Halle, Germany
4. Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands
5. School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, United States
6. Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York 14853, United States
7. Scottish Universities Physics Alliance, School of Physics and Astronomy, University of St. Andrews, Fife KY16 9SS, United Kingdom