1. Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, 4040 Linz, Austria
2. Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, 47057 Duisburg, Germany
3. Ernst Ruska-Centrum für Mikroskopie und Spektroskopie mit Elektronen, Forschungszentrum Julich GmbH, D-52425 Julich, Germany
4. Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk 660036, Russia
5. Stanford Synchrotron Radiation Laboratory, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
6. Department of Physics, University of California Santa Cruz, Santa Cruz, California 95064, USA