1. Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK.
2. IBM Research–Zurich, Säumerstrasse 4, 8803 Rüschlikon, Switzerland.
3. Institute of Physics, University of Muenster, Heisenbergstr, 11, 48149 Muenster, Germany.
4. Department of Engineering, University of Exeter, Exeter EX4 QF, UK.