1. Department of Physics; University of Basel; Klingelbergstrasse 82 4056 Basel Switzerland
2. Department of Physics, Chemistry and Biology; IFM; Linköping University; Linköping 58183 Sweden
3. Anorganisch-Chemisches Institut; Universität Heidelberg; Im Neuenheimer Feld 270 69120 Heidelberg Germany
4. Laboratory for Micro- and Nanotechnology; Paul Scherrer Institut; 5232 Villigen PSI Switzerland