National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093;
National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093;
National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093;
National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093;
Department of Physics, Hebei Normal University, Shijiazhuang 050091;
National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093;