1. Department of Material Science and Technology, University of Ruse “Angel Kanchev”, 8 Studentska Str., 7017 Ruse, Bulgaria
2. Roumen Tsanev Institute of Molecular Biology, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 21, 1113 Sofia, Bulgaria
3. Faculty of Physics, Sofia University “St. Kliment Ohridski”, 15 Tsar Osvoboditel Blvd, 1504 Sofia, Bulgaria
4. Institute of Electronics “Acad. Emil Djakov”, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria
5. Department of Mathematics, Informatics and Natural Sciences, Technical University of Garbovo, 4 H. Dimitar Str., 5300 Gabrovo, Bulgaria