1. Georgi Nadjakov Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., BG-1784 Sofia, Bulgaria
2. Institute of Physics, Faculty of Natural Sciences and Mathematics, Sts. Cyril and Methodius University, 3 Arhimedova Str., MKD-1000 Skopje, R. N. Macedonia