Design of Antiferromagnetic Second-Order Band Topology with Rotation Topological Invariants in Two Dimensions.

The existence of fractionally quantized topological corner charge serves as a key indicator for two-dimensional (2D) second-order topological insulators (SOTIs), yet it has not been experimentally observed in realistic materials. Here, based on effective model analysis and symmetry arguments, we propose a strategy for achieving SOTI phases with in-gap corner states in 2D systems with antiferromagnetic (AFM) order. We discover that the band topology originates from the interplay between intrinsic spin-orbital coupling and interlayer AFM exchange interactions. Using first-principles calculations, we show that the 2D AFM SOTI phase can be realized in (MnBi2Te4)(Bi2Te3)m films. Moreover, we demonstrate that the SOTI states are linked to rotation topological invariants under 3-fold rotation symmetry C3, resulting in fractionally quantized corner charge, i.e., n3|e| (mod e). Due to the great achievements in (MnBi2Te4)(Bi2Te3)m systems, our results providing reliable material candidates for experimentally accessible AFM SOTIs should draw intense attention.

Floquet Engineering of Nonequilibrium Valley-Polarized Quantum Anomalous Hall Effect with Tunable Chern Number.

Here, we propose that Floquet engineering offers a strategy to realize the nonequilibrium quantum anomalous Hall effect (QAHE) with tunable Chern number. Using first-principles calculations and Floquet theorem, we unveil that QAHE related to valley polarization (VP-QAHE) is formed from the hybridization of Floquet sidebands in the two-dimensional family MSi2Z4 (M = Mo, W, V; Z = N, P, As) by irradiating circularly polarized light (CPL). Through the tuning of frequency, intensity, and handedness of CPL, the Chern number of VP-QAHE is highly tunable and up to C = ±4, which attributes to light-induced trigonal warping and multiple-band inversion at different valleys. The chiral edge states and quantized plateau of Hall conductance are visible inside the global band gap, thereby facilitating the experimental measurement. Our work not only establishes Floquet engineering of nonequilibrium VP-QAHE with tunable Chern number in realistic materials but also provides an avenue to explore emergent topological phases under light irradiation.

Manipulating the Interfacial Band Bending For Enhancing the Thermoelectric Properties of 1T'-MoTe2 /Bi2 Te3 Superlattice Films.

Interfacial charge effects, such as band bending, modulation doping, and energy filtering, are critical for improving electronic transport properties of superlattice films. However, effectively manipulating interfacial band bending has proven challenging in previous studies. In this study, (1T'-MoTe2 )x (Bi2 Te3 )y superlattice films with symmetry-mismatch were successfully fabricated via the molecular beam epitaxy. This enables to manipulate the interfacial band bending, thereby optimizing the corresponding thermoelectric performance. These results demonstrate that the increase of Te/Bi flux ratio (R) effectively tailored interfacial band bending, resulting in a reduction of the interfacial electric potential from ≈127 meV at R = 16 to ≈73 meV at R = 8. It is further verified that a smaller interfacial electric potential is more beneficial for optimizing the electronic transport properties of (1T'-MoTe2 )x (Bi2 Te3 )y . Especially, the (1T'-MoTe2 )1 (Bi2 Te3 )12 superlattice film displays the highest thermoelectric power factor of 2.72 mW m-1 K-2 among all films, due to the synergy of modulation doping, energy filtering, and the manipulation of band bending. Moreover, the lattice thermal conductivity of the superlattice films is significantly reduced. This work provides valuable guidance to manipulate the interfacial band bending and further enhance the thermoelectric performances of superlattice films.

A carbon allotrope with twisted Dirac cones induced by grain boundaries composed of pentagons and octagons.

The grain boundaries (GBs) composed of pentagons and octagons (558 GBs) have been demonstrated to induce attractive transport properties such as Van Hove singularity (VHS) and quasi-one-dimensional metallic wires. Here, we propose a monolayer carbon allotrope which is formed from the introduction of periodic 558 GBs to decorate intact graphene, termed as PHO-graphene. The calculated electronic properties indicate that PHO-graphene not only inherits the previously superior characteristics such as Van Hove singularity and quasi-one-dimensional metallic wires, but also possesses two twisted Dirac cones near the Fermi level. Further calculation finds that the Berry phase is quantized to ± π at the two Dirac points, which is consistent with the distribution of the corresponding Berry curvature. The parity argument uncovers that PHO-graphene hosts a nontrivial band topology and the edge states connecting the two Dirac points are clearly visible. Our findings not only provide a reliable avenue to realize the abundant and extraordinary properties of carbon allotropes, but also offer an attractive approach for designing all carbon-based devices.

Enhancing the electronic structure of Ni-based electrocatalysts through N element substitution for the hydrogen evolution reaction.

The weak orbital coupling between Ni3N and H2O, caused by its interstitial structure and attenuated Ni-N interaction, is attributed to the high unoccupied d orbital energy of Ni3N. Consequently, the kinetics for water dissociation in the HER are slow. In this study, we effectively lowered the energy state of vacant d orbitals in Ni3N, which resulted in an exceptionally efficient HER. The as-synthesized Ni3N catalyst demonstrates an overpotential of 135 mV when subjected to a current density of 10 mA cm-2. The refined structural characterization suggests that the introduction of oxygen results in a reduction in electron densities surrounding the Ni sites. Furthermore, DFT calculations provide additional evidence that the electrocatalyst of Ni3N generates a greater number of lowest unoccupied orbitals (LUMOs) and improved alignment, thereby enhancing the adsorption and splitting of water. The notion of orbital-regulated electronic levels on Ni sites introduces a distinctive methodology for the systematic development of catalysts used in hydrogen evolution and other applications.

Pressure-induced transition from a Mott insulator to a ferromagnetic Weyl metal in La2O3Fe2Se2.

The insulator-metal transition in Mott insulators, known as the Mott transition, is usually accompanied with various novel quantum phenomena, such as unconventional superconductivity, non-Fermi liquid behavior and colossal magnetoresistance. Here, based on high-pressure electrical transport and XRD measurements, and first-principles calculations, we find that a unique pressure-induced Mott transition from an antiferromagnetic Mott insulator to a ferromagnetic Weyl metal in the iron oxychalcogenide La2O3Fe2Se2 occurs around 37 GPa without structural phase transition. Our theoretical calculations reveal that such an insulator-metal transition is mainly due to the enlarged bandwidth and diminishing of electron correlation at high pressure, fitting well with the experimental data. Moreover, the high-pressure ferromagnetic Weyl metallic phase possesses attractive electronic band structures with six pairs of Weyl points close to the Fermi level, and its topological property can be easily manipulated by the magnetic field. The emergence of Weyl fermions in La2O3Fe2Se2 at high pressure may bridge the gap between nontrivial band topology and Mott insulating states. Our findings not only realize ferromagnetic Weyl fermions associated with the Mott transition, but also suggest pressure as an effective controlling parameter to tune the emergent phenomena in correlated electron systems.

Bonding Heterogeneity Leads to Hierarchical and Ultralow Lattice Thermal Conductivity in Sodium Metavanadate.

Sodium metavanadate (NaVO3) is a promising low-cost candidate as a cathode material for sodium-ion batteries (SIBs) owing to its high cycle performance. Its thermal transport, although being a central factor limiting its practical applications, remains scarce. Herein, we comprehensively investigate the intrinsic thermal transport properties of the two phases of NaVO3 using the unified theory. Importantly, we identify a hierarchical thermal transport mechanism in NaVO3 and the significant contribution of diffusive thermal transport. With the combined two channels, we reveal that NaVO3 has the anisotropic and ultralow thermal conductivity κ, which is derived from the bonding heterogeneity with the coexistence of strong V-O bonds and weak Na-O bonds, implying the possibility of engineering the κ of SIBs by spatially tuning the sodium concentration distribution. Our work establishes a fundamental understanding of the intrinsic thermal transport of NaVO3 and provides guidance toward designing tunable thermal conductivity cathode materials for SIBs.

Dirac Fermions in Graphene with Stacking Fault Induced Periodic Line Defects.

The exploration of carbon phases with intact massless Dirac fermions in the presence of defects is critical for practical applications to nanoelectronics. Here, we identify by first-principles calculations that the Dirac cones can exist in graphene with stacking fault (SF) induced periodic line defects. These structures are width (n)-dependent to graphene nanoribbon and are thus termed as (SF)n-graphene. The electronic properties reveal that the semimetallic features with Dirac cones occur in (SF)n-graphene with n = 3m + 1, where m is a positive integer, and then lead to a quasi-one-dimensional conducting channel. Importantly, it is found that the twisted Dirac cone in the (SF)4-graphene is tunable among type-I, type-II, and type-III through a small uniaxial strain. The further stability analysis shows that (SF)n-graphene is thermodynamic stable. Our findings provide an artificial avenue for exploring Dirac Ffermions in carbon-allotropic structures in the presence of defects.