1. Research Center in the Physics of Matter and Radiation (PMR), Laboratoire Interdisciplinaire de Spectroscopie Electronique (LISE), University of Namur, B-5000 Namur, Belgium
2. NanoTecCenter Weiz Forschungsgesellschaft mbH, Franz-Pichler Straße 32, A-8160 Weiz, Austria
3. Institute of Solid State Physics, Graz University of Technology, A-8010 Graz, Austria
4. Institute of Applied Physics, Eberhard-Karls-Universität Tübingen, D-72076 Tübingen, Germany