Z. Gogua (Department of Physics, Georgian Technical University, 0175 Tbilisi, 77 Kostava St., Georgia, Georgia)
P. Kervalishvili (Department of Physics, Georgian Technical University, 0175 Tbilisi, 77 Kostava St., Georgia, Georgia)
G. Kantidze (Department of Physics, Georgian Technical University, 0175 Tbilisi, 77 Kostava St., Georgia, Georgia)