1. Solid State Section, Department of Physics, University of Athens, Panepistimiopolis, 15784 Zografos, Athens, Greece
2. Institute of Physics, Technical University of Szczecin, al. Piastów 17, 70-310 Szczecin, Poland
3. Faculty of Applied Mathematics and Physics, National Technical University of Athens, 15780 Athens, Greece
4. Laboratory of Inorganic Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, 54006 Thessaloniki, Greece
5. Applied Physics Section, Department of Physics, University of Athens, anepistimiopolis, 15784 Zografos, Athens, Greece