Affiliation: | (1) University of Chemical Technology and Metallurgy, 8 Kl. Ohridski Blvd., 1756 Sofia, Bulgaria;(2) South West University Neofit Rilski,, 66 Ivan Michailov Str., 2700 Blagoevgrad, Bulgaria;(3) Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria |