A nonsymmorphic-symmetry-protected hourglass Weyl node, hybrid Weyl node, nodal surface, and Dirac nodal line in Pd4X (X = S, Se) compounds

Hourglass Weyl and Dirac nodal line phonons, and drumhead-like and torus phonon surface states in orthorhombic-type KCuS

In parallel to electronic systems, the concept of topology has been extended to phonons, which has led to the birth of topological phonons. In this Communication, based on symmetry analysis and first-principles calculations, we propose that hourglass Weyl nodal line (HWNL) phonons and Dirac nodal line (DNL) phonons coexist in the phonon dispersion of a single material, KCuS, with a Pnma-type structure. The HWNLs and DNLs are relatively flat in frequency and well separated from other phonon bands. The drumhead-like phonon surface state and the torus phonon surface state appear at the [001] and [100] surfaces, respectively. The reason for this phenomenon is explained based on the Zak phase calculations. The DNL and HWNL phonons are symmetry-related and these phonons can also be observed in other realistic materials, such as BaSi₂, TiB and ZrSi, with a Pnma-type structure. Our Communication, for the first time, proves that phononic nodal lines with different types of degeneracies and different types of phonon surface states can be achieved in one single material. Thus, KCuS with a Pnma-type structure can be viewed as a good platform to investigate the entanglement between HWNL phonons and DNL phonons and to realize the drumhead-like and torus phonon surface states.

Theoretical realization of hybrid Weyl state and associated high catalytic performance for hydrogen evolution in NiSi

For electrochemical hydrogen evolution reaction (HER), developing high-performance catalysts without containing precious metals have been a major research focus in the present. Herein, we show the feasibility of HER catalytic enhancement in Ni-based materials based on topological engineering from hybrid Weyl states. Via a high-throughput computational screening from ∼140,000 materials, we identify that a chiral compound NiSi is a hybrid Weyl semimetal (WSM) showing bulk type-I and type-II Weyl nodes and long surface Fermi arcs near the Fermi level. Sufficient evidences verify that topological charge carriers participate in the HER process, and make the certain surface of NiSi highly active with the Gibbs free energy nearly zero (0.07 eV), which is even lower than Pt and locates on the top of the volcano plots. This work opens up a new routine to develop no-precious-metal-containing HER catalysts via topological engineering, rather than traditional defect engineering, doping engineering, or strain engineering.

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A new designation to catalytic enhancement via topological engineering is constructed

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An ideal hybrid Weyl catalyst NiSi is identified by a high-throughput material screening

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Sufficient evidences verify topological charge participates in catalytic enhancement

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NiSi has a higher theoretical hydrogen evolution activity than precious-metal Pt

Obvious Surface States Connecting to the Projected Triple Points in NaCl's Phonon Dispersion

With the development of computer technology and theoretical chemistry, the speed and accuracy of first-principles calculations have significantly improved. Using first-principles calculations to predict new topological materials is a hot research topic in theoretical and computational chemistry. In this work, we focus on a well-known material, sodium chloride (NaCl), and propose that the triple point (TP), quadratic contact triple point (QCTP), linear and quadratic nodal lines can be found in the phonon dispersion of NaCl with Fm3¯ m type structure. More importantly, we propose that the clear surface states connected to the projected TP and QCTP are visible on the (001) surface. It is hoped that further experimental investigation and verification for these properties as mentioned above.