1. College of Mathematics and Physics, Chongqing University of Posts and Telecommunications, Chongqing 400065, P. R. China
2. Institute of Physics, University of Tartu, Riia 142, Tartu 51014, Estonia
3. Institute of Physics, Polish Academy of Sciences, al. Lotników
32/46, 02-668 Warsaw, Poland
4. Institute of Physics, Kazimierz Wielki University, Weyssenhoffa 11, 85-072 Bydgoszcz, Poland