Spin-1 Dirac-Weyl fermions protected by bipartite symmetry

We propose that bipartite symmetry allows spin-1 Dirac-Weyl points, a generalization of the spin-1/2 Dirac points in graphene, to appear as topologically protected at the Fermi level. In this spirit, we provide methodology to construct spin-1 Dirac-Weyl points of this kind in a given 2D space group and get the classification of the known spin-1 systems in the literature. We also apply the workflow to predict two new systems, P3m1-9 and P31m-15, to possess spin-1 at K/K' in the Brillouin zone of hexagonal lattice. Their stability under various strains is investigated and compared with that of T-3, an extensively studied model of ultracold atoms trapped in optical lattice with spin-1 also at K/K'. (C) 2015 AIP Publishing LLC.