1. Faculty of Physics, University of Duisburg-Essen, 47057 Duisburg, Germany
2. Department of Physical Sciences, Chuka University, P.O. Box 109, 60400 Chuka, Kenya
3. Department of Metallurgical and Materials Engineering, Muğla Sıtkı Koçman University, 48000 Mugla, Turkey
4. Materials Science (Materialwissenschaft), Technische Universität, 64287 Darmstadt, Germany