1. a Institute of Technical Physics , Riga Technical University , 14/24 Azenes str., Riga , LV-1048 , Latvia
2. b Institute of Silicate Materials , Riga Technical University , 14/24 Azenes str., Riga , LV-1048 , Latvia
3. c Institute of Solid State Physics , University of Latvia , 8 Kengaraga str., Riga , LV-1063 , Latvia