1. Department of Mathematics and Computer Science, Freie Universität Berlin, 14195 Berlin, Germany;
2. Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany
3. Department of Chemistry and Center for Theoretical Biological Physics, Rice University, Houston, Texas 77005, USA;
4. Physics and Materials Science Research Unit, University of Luxembourg, 1511 Luxembourg, Luxembourg;
5. Department of Computer Science, Technical University Berlin, 10587 Berlin, Germany;
6. Max-Planck-Institut für Informatik, 66123 Saarbrücken, Germany
7. Department of Brain and Cognitive Engineering, Korea University, Seoul 136-713, South Korea
8. Department of Physics, Rice University, Houston, Texas 77005, USA