We theoretically study the three-dimensional topological semimetals with nodal surfaces protected by crystalline symmetries. Different from the well-known nodal-point and nodal-line semimetals, in these materials, the conduction and valence bands cross on closed nodal surfaces in the Brillouin zone. We propose different classes of nodal surfaces, both in the absence and in the presence of spinorbit coupling (SOC). In the absence of SOC, a class of nodal surfaces can be protected by spacetime inversion symmetry and sublattice symmetry and characterized by a Z2 index, while another class of nodal surfaces are guaranteed by a combination of nonsymmorphic two-fold screw-rotational symmetry and time-reversal symmetry. We show that the inclusion of SOC will destroy the former class of nodal surfaces but may preserve the latter provided that the inversion symmetry is broken. We further generalize the result to magnetically ordered systems and show that protected nodal surfaces can also exist in magnetic materials without and with SOC, given that certain magnetic group symmetry requirements are satisfied. Several concrete nodal-surface material examples are predicted via the first-principles calculations. The possibility of multi-nodal-surface materials is discussed.arXiv:1712.09773v2 [cond-mat.mes-hall]

Liquid metal forms a thin layer of oxide skin via exposure to oxygen and this layer could be exfoliated by mechanical delamination or gas-injection/solvent-dispersion. Although the room-temperature fabrication of two-dimensional (2D) oxide through gas-injection and water-dispersion has been successfully demonstrated, a synthetic protocol in nonaqueous solvent at elevated temperature still remains as a challenge. Herein we report the mass-production of amorphous 2D SnO x nanoflakes with Bi decoration from liquid Sn–Bi alloy and selected nonaqueous solvents. The functional groups of the solvents play a key role in determining the final morphology of the product and the hydroxyl-rich solvents exhibit the best control toward 2D SnO x . The different solvent-oxide interaction that facilitates this phase-transfer process is further discussed on the basis of DFT calculation. Finally, the as-obtained 2D SnO x is evaluated in electrocatalytic CO2 reduction with high faradaic efficiency (>90%) of formic acid and stable performance over 10 h.

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In electronic band structures, nodal lines may arise when two (or more) bands contact and form a one-dimensional manifold of degeneracy in the Brillouin zone. Around a nodal line, the dispersion for the energy difference between the bands is typically linear in any plane transverse to the line. Here, we perform an exhaustive search over all 230 space groups for nodal lines with higher-order dispersions that can be stabilized by crystalline symmetry in solid state systems with spin-orbit coupling and time reversal symmetry. We find that besides conventional linear nodal lines, only lines with quadratic or cubic dispersions are possible, for which the allowed degeneracy cannot be larger than two. We derive effective Hamiltonians to characterize the novel low-energy fermionic excitations for the quadratic and cubic nodal lines, and explicitly construct minimal lattice models to further demonstrate their existence. Their signatures can manifest in a variety of physical properties such as the (joint) density of states, magneto-response, transport behavior, and topological surface states. Using ab-initio calculations, we also identify possible material candidates that realize these exotic nodal lines. * Z.-M. Yu and W. Wu contributed equally to this work. †

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N-Doped hierarchical porous carbons with enhanced microporosity were prepared byin situintroduction of active site K+into MOF precursors.

A UiO-66-coated mesh membrane with micro- and nanostructures was designed and successfully fabricated on steel mesh through a simple solution immersion process, exhibiting hydrophilic and underwater superoleophobic properties. It displays an outstanding oil-water separation efficiency over 99.99% with a high water permeation flux of 12.7 × 10 L m h, so high purity water (with the residual oil content less than 4 ppm) can be readily obtained from such a simple mesh membrane from various oil-water mixtures. Its large-scale membrane production will facilitate its practical usage for the industrial and environmental water purification.