Institute of Theoretical Physics and Department of Physics, Shanxi University, Taiyuan 030006, China;
Department of Physics, State University of New York at Buffalo, Buffalo, New York 14260-1500, USA;
Institute of Theoretical Physics and Department of Physics, Shanxi University, Taiyuan 030006, China;
Department of Physics, State University of New York at Buffalo, Buffalo, New York 14260-1500, USA;