1. Center of Theoretical and Computational Physics and Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, China, School of Chemistry and School of Molecular and Microbial Biosciences, The University of Sydney, New South Wales 2006, Australia, Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, Anhui, P.R. China, and Center for the Physics of Materials and Department of Physics, McGill University, Montreal, PQ, H3A 2T8 Canada