1. Radiation Physics Laboratory, Institute of Solid State Physics, University of Latvia, Miera str.31, LV-2169, Salaspils, Latvia
2. Instituto de Fisica Teorica, Universidade Estadual Paulista, Rua Pamplona 145, 01405-900 Sao Paulo, Brasil
3. Institute of Theoretical Physics and Astronomy, A.Goštauto 12, 2600 Vilnius, Lithuania