1. School of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
2. School of Chemical Engineering and Laboratory of Physics, Faculty of Engineering, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
3. Institut für Festkörperphysik, Friedrich Schiller Universität Jena, 07743 Jena, Germany
4. Otto Schott Institute of Materials Research, Friedrich Schiller Universität Jena, Löbdergraben 32, 07743 Jena, Germany
5. A. N. Sevchenko Research Institute of Applied Physical Problems, Belarusian State University, 7 Kurchatova St., Minsk 220045, Belarus
6. Instituto Superior Técnico, P-2695-066 Bobadela LRS, Portugal