1. Institut für Physik, Martin-Luther-Universität Halle-Wittenberg, D-06099 Halle, Germany
2. Research Center Future Energy Materials and Systems of the University Alliance Ruhr, Faculty of Mechanical Engineering, Ruhr University Bochum, Universitätsstraße 150, D-44801 Bochum, Germany
3. Department of Physics and Materials Science, University of Luxembourg, L-1511 Luxembourg, Luxembourg
4. Department of Physics, West Virginia University, Morgantown, West Virginia 26506, United States