1. a Institute of Material Science, NCSR ‘Demokritos’ , 153 10 Ag. Paraskevi, Attiki , Greece
2. b Section of Solid State, Physics Department , Athens University , Panepistimiopolis, 157 71, Zografou , Greece
3. c Institute of Physics, Technical University of Szczecin , Al. Piastow 17, P1-70310, Poland