1. B. Verkin Institute for Low Temperature Physics and Engineering, National Academy of Sciences of Ukraine, 47 Nauky Ave., Kharkov 61103, Ukraine
2. Georgi Nadjakov Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tsarigradsko Shosse Blvd., Sofia 1784, Bulgaria
3. W. Trzebiatowski Institute of Low Temperatures and Structure Research, PAS Wroclaw, 1410 PL-50-050, Poland