1. National
Laboratory of Solid State Microstructure, Department of Physics, Nanjing University, Nanjing 210093, China
2. Condensed
Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, United States
3. Collaborative
Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
4. Center
for Statistical and Theoretical Condensed Matter Physics, Zhejiang Normal University, Jinhua 321004, China