1. Institute of Solid State Physics , University of Latvia , 8 Kengaraga Str., Riga , LV-1063 , Latvia
2. Institute of Physics , University of Tartu , 1 Ostwaldi, 50411 Tartu , Estonia
3. Riga Technical University , Institute of Inorganic Chemistry , 3/7 P. Valdena Str., LV-1048 , Riga , Latvia