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, Jan Dlugosz University, Armii Krajowej 13/15, PL-42200 Czestochowa, Poland
4. Institute
of Materials Science and Engineering, Technical University of Czestochowa, Al. Armii Krajowej 19, PL-42200 Czestochowa, Poland