1. Beijing National Laboratory for Condensed Matter Physics Institute of Physics Chinese Academy of Sciences 100190 Beijing P. R. China
2. School of Physical Sciences University of Chinese Academy of Sciences 100049 Beijing P. R. China
3. State Key Lab of New Ceramics and Fine Processing School of Materials Science and Engineering Tsinghua University 100084 Beijing P. R. China
4. Beijing National Center for Electron Microscopy and Laboratory of Advanced Materials Department of Materials Science and Engineering Tsinghua University 100084 Beijing P. R. China
5. College of Materials Science and Opto-Electronic Technology University of Chinese Academy of Sciences 100049 Beijing P. R. China
6. Condensed Matter Physics and Materials Science Division Brookhaven National Laboratory 11973 Upton NY USA