1. Physik-Institut, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
2. Electron Microscopy Center, Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland
3. Department of Quantum Matter Physics, University of Geneva, Quai E.-Ansermet 24, CH-1211 Geneva, Switzerland
4. Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
5. Laboratory of Ion Beam Physics, ETH Zürich, Otto-Stern-Weg 5, CH-8093 Zürich, Switzerland