1. Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 040 01 Košice, Slovakia
2. Department of Physics, Faculty of Electrical Engineering and Informatics, Technical University of Košice, Letná 9, 042 00 Košice, Slovakia
3. Wigner Center for Physics, Institute for Solid State Physics and Optics, H-1525 Budapest, P.O. Box 49, Hungary