1. Solid State Physics Chair, Ivan Franko National University of Lviv, Dragomanova str. 50, 79005 Lviv, Ukraine
2. Scientific-Technical and Educational Center of Low-Temperature Studies, Ivan Franko National University of Lviv, Dragomanova str. 50, 79005 Lviv, Ukraine
3. Institute of Experimental Physics, University of Wrocław, pl. M. Borna 9, 50204 Wrocław, Poland
4. V.E. Lashkaryov Institute of Semiconductor Physics NAS of Ukraine, pr. Nauki, 41, 03028 Kyiv, Ukraine