1. Institute of Materials Science and Applied Research, Vilnius University, Naugarduko 24, Vilnius 2006, Lithuania, Institute of Physics, A. Gostauto 12, Vilnius 2600, Lithuania, Department of General Physics and Spectroscopy, Vilnius University, Sauletekio 9, corp. 3, Vilnius 2054, Lithuania, and Institute of Physical Energetics, Latvian Academy of Sciences, Aizkraukles 21, LV-1006, Riga, Latvia