Department of Materials Science Zhejiang University Hangzhou 310027;
National Laboratory of Solid State Microstructures Nanjing University Nanjing 210008;
Department of Physics Nanjing University Nanjing 210008;
Department of Physics University of Science and Technology of China Hefei 230026;
National Laboratory of Solid State Microstructures Nanjing University Nanjing 210008;