Kovtun, G. P., National Scientific Center “Kharkov Institute of Physics and Technology”1, Akademicheskaya st., 61108, Kharkov, Ukraine, tel/fax 057-335-60-27V.N. Karazin Kharkov National University61022, Kharkov, Svoboda square, 4, tel/fax 057-335-28-33, Ukraine