1. Experimental Physics VI, Julius-Maximilians-University of Würzburg; D-97074 Würzburg Germany
2. Institute of Physics and Astronomy, University of Potsdam; Karl-Liebknecht-Str. 24-25 14476 Potsdam Germany
3. Bavarian Center for Applied Energy Research e.V. (ZAE Bayern); D-97074 Würzburg Germany