1. Technische Physik, Physikalisches Institut and Würzburg-Dresden Cluster of Excellence ct.qmat, Universität Würzburg, 97074Würzburg, Germany
2. Institute of Condensed Matter Theory and Optics, Friedrich-Schiller-University Jena, D-07743Jena, Germany
3. Institute of Physics, University of Oldenburg, 26129Oldenburg, Germany