1. Institute for Advanced Study, Tsinghua University, Beijing 100084, China.
2. Department of Physics, University of California, Berkeley, Berkeley, CA 94720, USA.
3. Department of Physics, Stanford University, Stanford, CA 94305, USA.
4. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory and Stanford University, Menlo Park, CA 94025, USA.
5. Institute of Scientific Computing, Faculty of Mathematics, Technische Universität Dresden, 01069 Dresden, Germany.
6. State Key Laboratory of Low Dimensional Quantum Physics, Tsinghua University, Beijing 100084, China.