Institution: | 1. Beijing National Laboratory for Condensed Matter Physics & Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China; 2. Department of Physics, University of Science & Technology of China, Hefei 230026, China; 3. Department of Physics, McCullough Building, Stanford University, Stanford, CA 94305-4045, USA & Center for Advanced Study, Tsinghua University, Beijing 100084, China |