1. Department of Physics, University of Ioannina, P.O. Box 1186, 451 10 Ioannina, Greece
2. Bergische Universität Wuppertal, 42119 Wuppertal, Germany
3. Institute of Physics, Johannes Gutenberg University, Mainz, Germany
4. Max Planck Institute for Polymer Research, 55128 Mainz, Germany