Tkachenko, V. I., National Science Center “Kharkov Institute of Physics and Technology” The National Academy of Sciences of Ukraine 61108, Kharkov,1, Akademicheskaya str., tel./fax 8-057-349-10-78V.N. Karazin Kharkiv National University 61022, Kharkov,4, Svobody sq., tel./fax 8-057-705-14-05, Ukraine