Two-dimensional pentagonal crystals and possible spin-polarized Dirac dispersion relations

Based on first-principles calculations we show that the two-dimensional pentagonal (pt) structures, the compositions of pt-BN2, pt-C, and pt-Fe2S, are stable. As a common feature, they are composed of 3 components: 2 stretched honeycomb sublattices and 1 square sublattice, conferring flexibility of tailoring the properties peculiar to the graphene. Although the Dirac dispersion relation is removed in metallic pt-BN2 and insulating pt-C due to the hybridization of two honeycomb sublattices, it survives in pt-Fe2S because of the suppression of such hybridization between different spins. As a result, in the dispersion relation of pt-Fe2S spin-polarized and anisotropic Dirac cones occur. We suggest that such type of dispersion relation can be used to produce spin-filter effect by applying electric bias in a specific direction.