1. a Physics Department , Rostov State University , 5 Zorge St., 344090 , Rostov on Don , Russia
2. b Laboratorium für Angewandte Physik,Gerhard-Mercator-Universität Duisburg , D-47048 , Duisburg , Germany
3. c Institute of Physics, University of Silesia , Uniwersytecka 4, PL-40–007 , Katowice , Poland