1. Department of Experimental Physics, Saarland University, D-66123 Saarbrücken, Germany, Humboldt University of Berlin, Institute of Mathematics, Unter den Linden 6, D-10099 Berlin, Germany, Weierstrass Institute for Applied Analysis and Stochastics (WIAS), Mohrenstrasse 39, D-10117 Berlin, Germany, Max-Planck-Institute for Metals Research, Heisenbergstrasse 3, D-70569 Stuttgart, Germany, and Institute for Theoretical and Applied Physics, University of Stuttgart, Pfaffenwaldring 57, D-70569 Stuttgart, Germany