1. Institute of Nanostructure and Solid State Physics; Universität Hamburg; Jungiusstraße 11 D-20355 Hamburg Germany
2. International Iberian Nanotechnology Laboratory; Av. Mestre Jose Veiga; Braga 4715 Portugal
3. IBM Research-Zurich; Säumerstrasse 4 8803 Rüschlikon Switzerland
4. Wigner Research Centre of Physics; Hungarian Academy of Sciences; Konkoly-Thege út 29-33 H-1121 Budapest Hungary
5. Depto. Física; Universidad de Oviedo; E-33006 Oviedo Spain
6. Leibniz Institute for Solid State and Material Research; Helmholtzstraße 20 D-01069 Dresden Germany