1. London Centre
for Nanotechnology, University College London, 17−19 Gordon Street,
London WC1H 0AH, United Kingdom
2. Department of Physics and Astronomy, University College London, Gower Street, London WC1E
6BT, United Kingdom
3. National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United
Kingdom
4. École Polytechnique, 91128 Palaiseau Cedex, France
5. Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R.
China
6. IBM Research Division, Zurich Research Laboratory, Säumerstrasse
4, 8803 Rüschlikon,
Switzerland
7. JPK Instruments AG, Bouchéstrasse 12, 12435 Berlin, Germany