1. Department of Physics, University of California at Berkeley, Berkeley, California 94720, United States
2. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. Institute of Theoretical Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
4. School of Physical Science and Technology, Lanzhou University, Lanzhou, Gansu 730000, China
5. Department of Chemistry and Laboratory for Advanced Materials, Stanford University, Stanford, California 94305, United States