Department of Physics Renmin University of China Beijing 100872 China;
Songshan Lake Materials Laboratory Dongguan 523808 China;
Institute of Physics Chinese Academy of Sciences Beijing 100190 China;
Department of Physics Renmin University of China Beijing 100872 China;
Songshan Lake Materials Laboratory Dongguan 523808 China;
Songshan Lake Materials Laboratory Dongguan 523808 China;
University of Virginia Charlottesville VA 22904 USA;
New York University New York NY 10003 USA;
Songshan Lake Materials Laboratory Dongguan 523808 China;
Institute of Physics Chinese Academy of Sciences Beijing 100190 China;
Department of Physics Renmin University of China Beijing 100872 China;
Songshan Lake Materials Laboratory Dongguan 523808 China;
