1. Department of Quantum Matter Physics, University of Geneva, 1211 Geneva, Switzerland
2. Electron Spectrometry and Microscopy Laboratory (LSME), Institute of Physics (IPHYS), École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
3. Physik-Institut, University of Zurich, 8057 Zurich, Switzerland