1. Institute of Physics II, University of Cologne, 50937 Cologne, Germany
2. Felix Bloch Institute for Solid State Physics, University of Leipzig, 04109 Leipzig, Germany
3. Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany
4. PGI-7, Forschungszentrum Jülich, 52428 Jülich, Germany
5. Institute of Electronic Materials (IWE2), RWTH Aachen University, 52062 Aachen, Germany
6. Andrew & Erna Viterbi Department of Electrical Engineering, Technion—Israel Institute of Technology, 3200003 Haifa, Israel