1. Department of Materials, ETH Zürich, 8093 Zürich, Switzerland
2. Department of Physics, University of Fribourg, 1700 Fribourg, Switzerland
3. Department of Chemistry and Physics of Materials University of Salzburg, 5020 Salzburg, Austria
4. Department of Materials Science and Engineering, Department of Physics, Cornell University, Ithaca, NY, 14850, USA