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.
Beryllium is a simple alkali earth metal, but has been the target of intensive studies for decades because of its unusual electron behaviors at surfaces. Puzzling aspects include (i) severe deviations from the description of the nearly free electron picture, (ii) anomalously large electron-phonon coupling effect, and (iii) giant Friedal oscillations. The underlying origins for such anomalous surface electron behaviors have been under active debate, but with no consensus. Here, by means of first-principle calculations, we discover that this pure metal system, surprisingly, harbors the Dirac node line (DNL) that in turn helps to rationalize many of the existing puzzles. The DNL is featured by a closed line consisting of linear band crossings and its induced topological surface band agrees well with previous photoemission spectroscopy observation on Be (0001) surface. We further reveal that each of the elemental alakali earth metals of Mg, Ca, and Sr also harbors the DNL, and speculate that the fascinating topological property of DNL might naturally exist in other elemental metals as well.Topological semimetals [1] represent new types of quantum matter, currently attracting widespread interest in condensed matter physics and materials science. Compared with normal metals, topological semimetals are distinct in two essential aspects: the crossing points of the energy bands occur at the Fermi level, and some of the crossing points consist of the monopoles in the lattice momentum space. Topological semimetals can be classified into three main categories, topological Dirac (TD) [2], topological Weyl (TW) [3] and Dirac node line (DNL) semimetals [4][5][6], respectively. In the former two cases of TD and TW, the monopoles form isolated points in lattice momentum space and novel surface states (i.e., surface Dirac cones and Fermi-arc states) were observed or suggested, such as TD-type Na 3 Bi [7][8][9][10] In the third class of DNL, the crossings between energy bands form a fully closed line nearly at the Fermi level in the lattice momentum space, drastically different from the isolated Dirac (or Weyl) points in the TD and TW. The projection of the Dirac node line into a certain surface would result in a closed ring in which the topological surface states (usually flat bands) can be expected to appear due to the non-trivial topological property of its bulk phase. According to the previous DNL modelings [4,5], the band crossings occur at zero energy with a constraint chiral symmetry, leading to the appearance of flat topologically protected surface bands. However, in a real crystal the chiral symmetry of a band structure is not exact, thereby suggesting that the DNL does not generally occur at a constant energy and the DNL-induced topological surface bands are not flat either. Recently, this type of DNL states has been predicted in several cases of 3D carbon graphene allotropes [19] The metal of beryllium, which crystallizes in the hcp structure (see Fig. 1a), is a simple sp-bonded metal. Be is unusual in three aspects. F...
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