[1] Department of Physics, Rostock University, D-18051 Rostock, Germany
[2] Inst. of Math. and Computer Science, University of Latvia, LV-1459 Riga, Latvia
[3] General Physics Institute, Russian Academy of Sciences, Moscow, 119991, Russia
[4] Theoretical Physics Department, Institute of Single Crystals, Natl. Academy of Science of Ukraine, 61001 Kharkov, Ukraine