1. Département de Chimie Physique, Université de Genève, 1211 Genève, Switzerland
2. Institute of Condensed Matter Theory and Solid State Optics, Abbe Center of Photonics, Friedrich-Schiller-Universität Jena, D-07743 Jena, Germany
3. Institute of Nanotechnology, Karlsruhe Institute of Technology, D-76021 Karlsruhe, Germany
4. Institute of Theoretical Solid State Physics, Karlsruhe Institute of Technology, D-76131 Karlsruhe, Germany