1. Physical Department; Ivan Franko National University of Lviv; Dragomanova Str. 50 Lviv 79005 Ukraine
2. Scientific-Technical and Educational Center of Low-Temperature Studies; Ivan Franko National University of Lviv; Dragomanova Str. 50 Lviv 79005 Ukraine
3. Institute de Physique et de Chemie des Matériaux de Strasbourg; UMR 7504 CNRS-ULP, 23 rue du Loess, BP 43 67034 Strasbourg Cedex 2 France