1. Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 7610001, Israel.
2. Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany.
3. Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
4. John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA.
5. Department of Physics, Harvard University, Cambridge, MA 02138, USA.