1. Department of Physics, Gebze Technical University, 41400 Kocaeli, Turkey
2. Department of Physics, Yildiz Technical University, 34210 İstanbul, Turkey
3. Institute of Physics, Martin Luther University Halle-Wittenberg, 06120 Halle (Saale), Germany
4. Department of Materials Science, School of Natural Sciences, University of Patras, 26504 Patra, Greece