1. Faculty of Physics and Center for Nanointegration (CENIDE), University of Duisburg-Essen 1 , 47057 Duisburg, Germany
2. Ernst Ruska Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute, Forschungszentrum Jülich GmbH 2 , 52425 Jülich, Germany
3. Advanced Light Source, Lawrence Berkeley National Laboratory 3 , Berkeley, California 94720, USA
4. 6 Department of Physics, University of California Berkeley, Berkeley, California 94720, USA
5. Institut für Halbleiter- und Festkörperphysik, Johannes Kepler University 4 , 4040 Linz, Austria
6. SLAC National Accelerator Laboratory, 5 Menlo Park, California 94025, USA
7. 7 Department of Material Sciences and Engineering, Stanford University, Stanford, California 94305, USA
8. 8 Department of Physics, University of California Santa Cruz, Santa Cruz, California 95064, USA