1. Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, U.K.
2. Key Lab of Artificial Micro- and Nano-Structures of Ministry of Education of China, School of Physics and Technology, Wuhan University, Wuhan 430072, P.R. China
3. Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, U.K.
4. Theory and Simulation of Materials (THEOS), École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
5. Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, U.K.
6. Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, U.K.
7. Oden Institute for Computational Engineering and Sciences, University of Texas at Austin, Austin, Texas 78712, United States
8. Department of Physics, University of Texas at Austin, Austin, Texas 78712, United States