1. Institute of Physics and Center for Nanotechnology, University of Münster, D-48149 Münster, Germany
2. Institute of Solid State Theory, University of Münster, D-48149 Münster, Germany
3. Institut für Theoretische Physik, Nichtlineare Optik und Quantenelektronik, Technische Universität Berlin, D-10623 Berlin, Germany
4. Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
5. Materials Science Factory, Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Madrid E-28049, Spain