1. 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
2. Friedrich-Schiller-University Jena, 07743 Jena, Germany
3. Department of Physics, West Virginia University, Morgantown, WV 26506, USA
4. Department of Physics and Materials Science, University of Luxembourg, 162a avenue de la Faïencerie, L-1511 Luxembourg, Luxembourg