1. Institute of Experimental Physics I, Justus-Liebig-University Gießen, Heinrich-Buff-Ring 16, 35392 Gießen, Germany
2. Center for Materials Research (ZfM), Justus-Liebig-University Gießen, Heinrich-Buff-Ring 16, 35392 Gießen, Germany
3. Istituto Officina dei Materiali (CNR-IOM), Area Science Park Strada Statale 14 km 163,5, 34149 Basovizza, Trieste, Italy
4. Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa
5. Dipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo, 1, 35131 Padova, Italy
6. Institute of Physical Chemistry, Justus-Liebig-University Gießen, Heinrich-Buff-Ring 17, 35392 Gießen, Germany