1. Institute for Microbiology and Genetics; Georg-August-University Göttingen; Justus-von-Liebig Weg 11 37077 Göttingen Germany
2. Department of Physics and Center for Nanoscience; LMU Munich; Amalienstrasse 54 80799 Munich Germany
3. Current address: Saarland University, Theoretical Physics; Campus E2 6 66123 Saarbrücken Germany