1. Department of Physics, AlbaNova University Center, Stockholm University, SE-10691 Stockholm, Sweden
2. Institute of Physics, Johannes Gutenberg University Mainz, 55128 Mainz, Germany
3. Institute of Physical Chemistry, University of Innsbruck, A-6020 Innsbruck, Austria
4. Max-Planck-Institute for Polymer Research, 55128 Mainz, Germany