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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Although doping with appropriate heteroatoms is a powerful way of increasing visible light absorption of wide-bandgap metal oxide photocatalysts, the incorporation of heteroatoms into the photocatalysts usually leads to the increase of deleterious recombination centers of photogenerated charge carriers. Here, a conceptual strategy of increasing visible light absorption without causing additional recombination centers by constructing an ultrathin insulating heterolayer of amorphous boron oxynitride on wide-bandgap photocatalysts is shown. The nature of this strategy is that the active composition nitrogen in the heterolayer can noninvasively modify the electronic structure of metal oxides for visible light absorption through the interface contact between the heterolayer and metal oxides. The photocatalysts developed show significant improvements in photocatalytic activity under both UV-vis and visible light irradiation compared to the doped counterparts by conventional doping process. These results may provide opportunities for flexibly tailoring the electronic structure of metal oxides.