1. State Key Laboratory for Mesoscopic Physics and Department of PhysicsPeking University Beijing 100871 P. R. China
2. State Key Laboratory of Heavy Oil ProcessingInstitute of New EnergyCollege of Chemical EngineeringChina University of Petroleum (East China) Qingdao 266580 P.R. China
3. School of Physics and Electronic Information EngineeringEngineering Research Center of Nanostructure and Functional MaterialsNingxia Normal University Guyuan Ningxia 756000 P. R. China
4. Collaborative Innovation Center of Quantum Matter, Beijing 100871 P. R. China