1. Materials Science and Technology Division Oak Ridge National Laboratory Oak Ridge TN 37831 USA
2. School of Materials Science & Engineering Beihang University Beijing 100191 China
3. Department of Materials Science & NanoEngineering Rice University Houston TX 77005 USA
4. Department of Physics and Astronomy Vanderbilt University Nashville TN 37235 USA
5. School of Physical Sciences and CAS Center for Excellence in Topological Quantum Computation University of Chinese Academy of Sciences Beijing 100049 China
6. National Synchrotron Radiation Laboratory CAS Center for Excellence in Nanoscience University of Science and Technology of China Hefei Anhui 230029 China