Institute of Solid State Physics, University of Latvia, 8 Kengaraga Str., Riga, LV-1063, LATVIA
A. Fedotovs
Institute of Solid State Physics, University of Latvia, 8 Kengaraga Str., Riga, LV-1063, LATVIA|Faculty of Physics and Mathematics, University of Latvia, 8 Zellu Str., Riga, LV-1010, LATVIA
U. Rogulis
Institute of Solid State Physics, University of Latvia, 8 Kengaraga Str., Riga, LV-1063, LATVIA|Faculty of Physics and Mathematics, University of Latvia, 8 Zellu Str., Riga, LV-1010, LATVIA