1. National Center of Competence in Research on “Nanoscale Science”, Institute of Physics, Klingelbergstrasse 82, CH-4056 Basel, Switzerland, Department of Physics, McGill University, 3600 University Street, Montreal PQ, H3A 2T8 Canada, Institute of Physics, Klingelbergstrasse 82, CH-4056 Basel, Switzerland, and IBM Research Division, Zurich Research Laboratory, CH-8803 Rüschlikon, Switzerland