1. Department of Physics, Adnan Menderes University, Aydin 09100, Turkey
2. Department of Electrical and Electronic Engineering, Bilkent University, Ankara 06800, Turkey
3. Department of Physics, Kırıkkale University, Kırıkkale 71450, Turkey
4. Physik Department E20, Technische Universität München, Garching 85748, Germany
5. Department of Physics, Bilkent University, Ankara 06800, Turkey