1. Quantum Solid State Physics, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, Box 2414, BE-3001 Leuven, Belgium
2. College of Engineering, Swansea University, Bay Campus, Fabian Way, Swansea SA1 8EN, U.K.
3. Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute, Forschungszentrum Jülich GmbH, 52428 Jülich, Germany
4. Department of Chemistry and Materials Science, Aalto University, P.O. Box 16100, FI-00076 Aalto, Finland