1. Center for Nanoscale Science and Department of Physics, Pennsylvania State University, University Park, PA 16802–6300, USA.
2. State Key Laboratory of Advanced Welding and Joining and Research Center of Flexible Printed Electronic Technology, Harbin Institute of Technology, Shenzhen 518055, People’s Republic of China.
3. Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
4. Francis Bitter Magnet Laboratory and Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
5. Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA.
6. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.