1. Dep. of Theoretical Electrotechnics and Electrical Measurement, TU, Park Komenského 3, SK-043 89 Košice, Slovakia
2. Institute of Experimental Physics, SAS, SK-043 53 Košice, Slovakia
3. Institute of Physics, A. Mickiewicz University, PL-61-614 Poznań, Poland
4. , Húskova 61, 041 11 Košice, Slovakia