1. Institute of Physics, University of Tartu, W. Ostwaldi 1, 50411 Tartu, Estonia
2. Institute of Physical Energetics, Krivu 11, LV-1006 Riga, Latvia
3. Institute of Solid State Physics, University of Latvia, Kengaraga 8, LV-1063 Riga, Latvia
4. Department of Life Science & Technology, Laboratory of Biochemistry, University for Applied Sciences, 10318 Berlin, Germany