1. Institute of Physics and nanoMA, Chemnitz University of Technology, D-09107 Chemnitz, Germany, Fraunhofer Institute IZM, D-09126 Chemnitz, Germany, Institute of Experimental Physics I, Leipzig University, D-04103 Leipzig, Germany, Max Planck Institute for Astronomy, D-69117 Heidelberg, Germany, and Institute of Solid State Physics, University of Jena, D-07743 Jena, Germany