1. Laboratory of Solid-State Physics and Magnetism; KU Leuven; Celestijnenlaan 200D 3001 Leuven Belgium
2. Department of Physics and the Quantum Theory Project; University of Florida; Gainesville FL 32611 USA
3. Research Group on Carbon Nanostructures (CARBONNAGe); University of Namur; Rue de Bruxelles 61 5000 Namur Belgium