1. State Key Laboratory
of Low Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084, P. R. China
2. L’Orme des Merisiers, Synchrotron SOLEIL, Saint Aubin-BP 48, 91192 Gif sur Yvette Cedex, France
3. Collaborative
Innovation Center of Quantum Matter, Beijing 100084, P. R. China