1. Department of Physics, Middle East Technical University, Dumlupinar Blv 1, 06800 Ankara, Turkey
2. Dipartimento di Scienze Chimiche e Farmaceutiche, Università degli Studi di Trieste, Via L. Giorgieri 1, I-34127 Trieste, Italy
3. Department of Physics, Bilkent University, 06800 Ankara, Turkey