1. Max-Planck-Institut für Festkörperforschung, D-70569 Stuttgart, Germany, INFM-CNR DEMOCRITOS Theory@Elettra group and SISSA Scuola Internazionale Superiore di Studi Avanzati, I-34014 Trieste, Italy, Institute of Nanotechnology, Research Center Karlsruhe, D-76021 Karlsruhe, Germany, and Institut de Physique des Nanostructures, Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland