1. Institute of Physics, Goethe-University Frankfurt, 60438 Frankfurt am Main, Germany
2. Institute of Materials Science, Technical University of Darmstadt, 64287 Darmstadt, Germany
3. Institute for Applied Materials Research, Bielefeld University of Applied Science, 33619 Bielefeld, Germany
4. Faculty of Physics, Bielefeld University, 33501 Bielefeld, Germany