1. University of Greifswald; Institute of Physics; Felix-Hausdorff-Str. 6 17489 Greifswald Germany
2. Faculty of Science; University of South Bohemia; Branisovska 31 37005 Ceske Budejovice Czech Republic
3. Institute of Physics; Academy of Science of the Czech Republic; Na Slovance 2 182 21 Prague Czech Republic
4. Faculty of Mathematics and Physics; Charles University in Prague; V Holesovickach 2 180 00 Prague Czech Republic