1. Quantum Solid-State Physics, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
2. Departamento de Física, Universidade Federal de São Carlos, 13565-905 São Carlos, São Paulo, Brazil
3. Imec, Kapeldreef 75, 3001 Leuven, Belgium
4. Experimental Physics of Nanostructured Materials, Department of Physics, Université de Liège, Allée du 6 Août 19, B-4000 Sart Tilman, Belgium