1. Institute of Physics, University of Potsdam, Am Neuen Palais 10, D-14469 Potsdam, Germany, Institute of Physics, Humboldt University Berlin, Newtonstrasse 15, D-12489 Berlin, Germany, and Institute of Polymer Research, GKSS Forschungszentrum Geesthacht GmbH, Kantstrasse 55, D-14513 Teltow, Germany