1. Institut für Physik, Martin-Luther-Universität, Halle-Wittenberg, D-06099 Halle/Saale, Germany
2. Physics Department, Tbilisi State University, 3 Chavchavadze, 0128 Tbilisi, Georgia
3. Faculty of Mathematics and Natural Sciences, University of Rzeszow, Pigonia 1, 35-310 Rzeszow, Poland