Belarusian State University of Informatics and Radioelectronics, Physics Department, P. Brovki Str., 6, 220013 Minsk, Belarus; Fax: +37517 2932333
Belarusian State University of Informatics and Radioelectronics, Physics Department, P. Brovki Str., 6, 220013 Minsk, Belarus; Fax: +37517 2932333