Department of Physics, Tianjin University, Tianjin 300072, Peoples Republic of China. ctzhang@tju.edu.cn|Department of Physics, Tianjin University, Tianjin 300072, Peoples Republic of China. ctzhang@tju.edu.cn|Department of Physics, Tianjin University, Tianjin 300072, Peoples Republic of China. ctzhang@tju.edu.cn