1. Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
2. Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
3. Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44, 3001 Leuven, Belgium