1. Institut für Physik, Universität Greifswald,
Felix-Hausdorff-Str. 6, 17489 Greifswald, Germany
2. Institute of Physics, Academy of Science of the Czech Republic, Na Slovance 2, 18221 Prague, Czech Republic
3. Faculty of Science, University of South Bohemia, Branisovska 31, 37005 Ceske Budejovice, Czech Republic