1. Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 182 21 Prague 8, Czech Republic
2. Department of Physics of Materials, Charles University, Ke Karlovu 5, 121 16 Prague, Czech Republic
3. Institute of Physics, P. J. Šafárik University, Park Angelinum 9, 040 01 Košice, Slovakia
4. Institute of Experimental Physics, SAS, Watsonova 47, 040 01 Košice, Slovakia