1. Department of Chemical and Biological Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
2. Department of Applied Physics, Hebrew University, Jerusalem 91904, Israel.
3. IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120, USA.
4. Max Planck Institute for Microstructure Physics, Halle (Saale) D-06120, Germany.
5. Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel.
6. Institute of Physics Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warszawa, Poland.