1. Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussée, 1784 Sofia, Bulgaria
2. Institute of Electronics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussée, 1784 Sofia, Bulgaria
3. Physics and Technology Faculty, University of Plovdiv “Paisii Hilendarski”, 24 Tsar Asen Straße, 4000 Plovdiv, Bulgaria
4. Faculty of Applied Mathematics and Computer Science, Technical University of Sofia, 8 Kliment Ohridski Boulevard, 1000 Sofia, Bulgaria