Department of Physics, College of Science, Yanbian University, Yanji 133002;
Department of Physics, College of Science, Yanbian University, Yanji 133002;
Department of Physics, College of Science, Yanbian University, Yanji 133002;
Department of Physics, College of Science, Yanbian University, Yanji 133002;
Center for the Condensed-Matter Science and Technology, Harbin Institute of Technology, Harbin 150001;
Department of Physics, College of Science, Yanbian University, Yanji 133002;
Center for the Condensed-Matter Science and Technology, Harbin Institute of Technology, Harbin 150001;
BK21 Program Physics and Department of Physics, College of Natural Science, Chungbuk National University, Cheonju, Chungbuk 361-763, Republic of Korea;