National Laboratory of Solid State Microstructures and College of Engineering and Applied Sciences Nanjing University Nanjing 210093 China;
National Laboratory of Solid State Microstructures and College of Engineering and Applied Sciences Nanjing University Nanjing 210093 China;
National Laboratory of Solid State Microstructures and College of Engineering and Applied Sciences Nanjing University Nanjing 210093 China;
National Laboratory of Solid State Microstructures and College of Engineering and Applied Sciences Nanjing University Nanjing 210093 China;
Department of Physics University of Arkansas Fayetteville Arkansas 72701 USA;