1. Institute of Solid State Physics, University of Latvia, Kengaraga Str. 8, LV-1083 Riga, Latvia
2. Natural Sciences and Computer Technologies Department, Information Systems, Management Institute, Lomonosov Str. 1, Bld 6, LV-1019 Riga, Latvia
3. INFN-Laboratori Nazionali di Frascati, Via Enrico Fermi 40, Frascati, Italy