1. Centre of Low Temperature Physics, Institute of Experimental Physics SAS, Watsonova 47, 04001 Košice, Slovakia
2. Faculty of Electrical Engineering and Informatics, Technical University, Letná 9, 04200 Košice, Slovakia
3. Institute for Problems of Material Science, NASU, Krzhyzhanovsky 3, 03680 Kiev, Ukraine
4. Helmholtz–Zentrum Berlin, Glienicker Str. 100, 14109 Berlin, Germany