Three-dimensional (3D) Dirac point, where two Weyl points overlap in momentum space, is usually unstable and hard to realize. Here we show, based on the first-principles calculations and effective model analysis, that crystalline A3Bi (A=Na, K, Rb) are Dirac semimetals with bulk 3D Dirac points protected by crystal symmetry. They possess nontrivial Fermi arcs on the surfaces and can be driven into various topologically distinct phases by explicit breaking of symmetries. Giant diamagnetism, linear quantum magnetoresistance, and quantum spin Hall effect will be expected for such compounds.

Stabilizing monolayer nitrides with silicon: Transition metal carbides and nitrides are nonlayered materials that in monolayer form have potentially useful electronic and chemical properties. These monolayers are usually made by chemical etching that produces flakes with surface defects that have poor air and water stability. Hong et al. report that introducing silicon during chemical vapor deposition growth of molybdenum nitride passivates the surface and prevents island formation. Centimeter-scale monolayer films of the semiconductor MoSi ₂ N ₄ form as a MoN ₂ layer sandwiched by two Si-N bilayers. These layers possess high mechanical strength and ambient stability. Science , this issue p. 670

The search for new two-dimensional monolayers with diverse electronic properties has attracted growing interest in recent years. Here, we present an approach to construct MA₂Z₄ monolayers with a septuple-atomic-layer structure, that is, intercalating a MoS₂-type monolayer MZ₂ into an InSe-type monolayer A₂Z₂. We illustrate this unique strategy by means of first-principles calculations, which not only reproduce the structures of MoSi₂N₄ and MnBi₂Te₄ that were already experimentally synthesized, but also predict 72 compounds that are thermodynamically and dynamically stable. Such an intercalated architecture significantly reconstructs the band structures of the constituents MZ₂ and A₂Z₂, leading to diverse electronic properties for MA₂Z₄, which can be classified according to the total number of valence electrons. The systems with 32 and 34 valence electrons are mostly semiconductors. Whereas, those with 33 valence electrons can be nonmagnetic metals or ferromagnetic semiconductors. In particular, we find that, among the predicted compounds, (Ca,Sr)Ga₂Te₄ are topologically nontrivial by both the standard density functional theory and hybrid functional calculations. While VSi₂P₄ is a ferromagnetic semiconductor and TaSi₂N₄ is a type-I Ising superconductor. Moreover, WSi₂P₄ is a direct gap semiconductor with peculiar spin-valley properties, which are robust against interlayer interactions. Our study thus provides an effective way of designing septuple-atomic-layer MA₂Z₄ with unusual electronic properties to draw immediate experimental interest.