1. Materials Research and Technology Department, Luxembourg Institute of Science and Technology, 41 rue du Brill, L-4422 Belvaux, Luxembourg
2. Department of Physics and Materials Science, University of Luxembourg, 41 rue du Brill, L-4422 Belvaux, Luxembourg
3. DQMP – University of Geneva, École de Physique, 24, Quai Ernest-Ansermet, CH-1211 Geneva, Switzerland
4. Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 4, 8093 Zurich, Switzerland
5. Physics Department and Institute for Nanoscience and Engineering University of Arkansas, Fayetteville, Arkansas 72701, USA
6. Physik-Institut, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland