1. Physics Department, Donetsk State University, 340055 Donetsk, Ukraine
2. Institute of Physics, University at Białystok, Lipowa 41, 15-424 Białystok, Poland
3. Institute of Physics, Academy of Sciences of Czech Republic, Na Slovance 2, 18040 Praha 8, Czech Republic
4. Lab. de Phys. des Solides, URA CNRS 02, Université Paris-Sud, 91405 Orsay, France