1. Institute of Solid State Physics; Bulgarian Academy of Sciences; Tzarigradsko Chaussee 72 1784 Sofia Bulgaria
2. Institute of Photonics and Electronics CAS; Chaberska 57 182 51 Prague 8 Czech Republic
3. Laboratory of Inorganic Materials; Joint Workplace of the University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, and the Institute of Rock Structure and Mechanics of the CAS; v.v.i., V Holešovičkách 41 182 09 Prague 8 Czech Republic
4. Laboratoire d'Electronique Quantique; USTHB Bab-Ezzouar; 16111 Alger Algeria
5. Institute of Solid State Physics; University of Latvia; Kengaraga 8 1063 Riga Latvia