College of Physics, Hebei Normal University, Shijiazhuang 050016;
School of Materials Science and Engineering, Beijing University of Science and Technology, Beijing 100083;
CCAST (World Laboratory), PO Box 8730, Beijing 100080;
College of Physics, Hebei Normal University, Shijiazhuang 050016;
CCAST (World Laboratory), PO Box 8730, Beijing 100080;
