1. Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, Erwin-Schrödinger-Straße 46, 67663 Kaiserslautern, Germany
2. Experimentelle Physik VI, Technische Universität Dortmund, 44221 Dortmund, Germany
3. Graduate School of Excellence Materials Science in Mainz, Erwin-Schrödinger-Straße 46, 67663 Kaiserslautern, Germany