1 Department of Physics, The University of Texas at Austin, Austin, TX, USA.
2 Institute for Theoretical Physics, Vienna University of Technology, Vienna, Austria.
3 State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China.
4 National Institute for Material Science, Tsukuba, Japan.
5 State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China. phtan@semi.ac.cn.
6 Institute for Theoretical Physics, Vienna University of Technology, Vienna, Austria. florian.libisch@tuwien.ac.at.
7 Department of Physics, The University of Texas at Austin, Austin, TX, USA. elaineli@physics.utexas.edu.