National Laboratory for Superlattices and Microstructures Institute of Semiconductors Chinese Academy of Sciences Beijing 100083Department of Physics Hebei Normal University Shijiazhuang 050016;
National Laboratory for Superlattices and Microstructures Institute of Semiconductors Chinese Academy of Sciences Beijing 100083Department of Physics Hebei Normal University Shijiazhuang 050016;
National Laboratory for Superlattices and Microstructures Institute of Semiconductors Chinese Academy of Sciences Beijing 100083Department of Physics Hebei Normal University Shijiazhuang 050016;
National Laboratory for Superlattices and Microstructures Institute of Semiconductors Chinese Academy of Sciences Beijing 100083;