He Ma,a Yu Wang,a Rong Lu,a Fangrui Tan,a Yulan Fu,a Guang Wang,b Dayong Wang,a Kai Liu,c Shoushan Fan,b Kaili Jiangb and Xinping Zhang*a
a College of Applied Sciences, Beijing University of Technology, Beijing 100124, P. R. China
b State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua-Foxconn Nanotechnology Research Center, Department of Physics, Collaborative Innovation Center of Quantum Matter, Tsinghua University, Beijing 100084, P. R. China
c State Key Laboratory of New Ceramics and Fine Processing, School of Material Science and Engineering, Tsinghua University, Beijing 100084, P. R. China