1. Experimentalphysik 2, Universität Augsburg, D-86159 Augsburg, Germany
2. Department Materials Research and Technology, Luxembourg Institute of Science and Technology, 41 Rue du Brill, L-4422 Belvaux, Luxembourg
3. Physics and Materials Science Research Unit, University of Luxembourg, 41 Rue du Brill, L-4422 Belvaux, Luxembourg