1. Department of Physics (ECS), Technical University of Munich, James-Franck-Straße 1, 85748 Garching, Germany
2. Cluster of Excellence E-Conversion, Schellingstraße 4, 80799 Munich, Germany
3. Max Planck-EPFL Laboratory for Molecular Nanoscience and Technology, Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
4. Institute of Physics (IPHYS), Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland