1. Catalan Institute of Nanotechnology (ICN), UAB Campus, E-08193 Barcelona,
Spain
2. European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble, France
3. Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), D-76344 Eggenstein-Leopoldshafen,
Germany
4. Institute de Physique
et Chimie
de Matériaux de Strasbourg (IPCMS), UMR 7504, CNRS-Université de Strasbourg, F-67034 Strasbourg,
France
5. Max-Planck-Institut für Festkörperforschung, D-70569 Stuttgart,
Germany
6. Institució Catalana de Recerca i Estudis Avançats (ICREA), E-08100
Barcelona, Spain
7. Departament
de Física, Universitat Autònoma de Barcelona, E-08193 Barcelona,
Spain