1. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan.
2. Department of Physics, Princeton University, Princeton, NJ 08544, USA.
3. National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China.
4. Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA.
5. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
6. CAS Center for Excellence in Topological Quantum Computation, University of Chinese Academy of Sciences, Beijing 100190, China.