Monolayer Cu2Se: a topological catalysis in CO2electroreduction.

This investigation provides a comprehensive exploration into the intricate interplay between topological surface states (TSS) and catalytic performance in two-dimensional (2D) materials, with specific emphasis on monolayer Cu2Se. Leveraging the unique characteristics of nodal loop semimetals (NLSMs), we delve into the precise influence of TSS on catalytic activity, particularly in the domain of CO2electrochemical reduction. Our findings illuminate the central role played by these TSS, arising from the underlying NLSM framework, in sculpting catalytic efficiency. The length of these surface states emerges as a critical determinant of surface density of states (DOSs), a fundamental factor governing catalytic behavior. Extension of these surface states correlates with heightened surface DOSs, yielding lower Gibbs free energies and consequently enhancing catalytic performance, particularly in the electrochemical reduction of CO2. Moreover, we underscore the profound importance of preserving symmetries that protect the nodal loop. The disruption of these symmetries is found to result in a significant degradation of catalytic efficacy, underscoring the paramount significance of topological features in facilitating catalytic processes. Therefore, this study not only elucidates the fundamental role of TSS in dictating the catalytic performance of topological 2D materials but also paves the way for harnessing these unique attributes to drive sustainable and highly efficient catalysis across a diverse spectrum of chemical processes.

Drumhead surface states promoted hydrogen evolution reactions in type-II nodal-line topological catalyst Mg3Bi2.

An excellent catalyst generally meets three indicators: high electron mobility, high surface density of states and low Gibbs free energy (ΔG) [H. Luo et al. Nat. Rev. Phys., 2022, 4, 611-624]. Recent studies have confirmed that topological materials exhibit more advantages than conventional precious metals with regard to the above-mentioned indicators. Herein, based on DFT calculations and symmetry analysis, we discovered for the first time that the topological surface states of Mg3Bi2 with a Kagome lattice promote hydrogen evolution reactions (HERs). In particular, there exists a snake-like type-II nodal loop (NL), located on kz = 0 plane in Mg3Bi2. Besides, the NL forms a topologically protected drumhead surface state on the (001) surface. It was found that the ΔG (0.176 eV) value of the (001) surface is comparable to that of the precious metal Pt. Then, through hole doping and strain regulation, it was found that the catalytic activity of Mg3Bi2 is closely related to the drumhead surface state formed by NL. With the above-mentioned results, this study not only provides a promising candidate material for hydrogen electrolysis, but also deepens our understanding of the dominant factors of NL semimetals for the catalytic activity.

Realization of high-order topological phase transition in 2D metal-organic frameworks.

In two-dimensional (2D) scale, controllable topological phase transition between a conventional topological quantum state and a higher-order one has been a challenge currently. Herein, based on first-principles, we report 2D metal-organic frameworks (MOFs) are ideal choice for realizing such topological phase transition. Taking MOF candidate Pd3(C6S6)2as an example, a semimetallic band structure is present at the equilibrium state. Under moderate compressive strain, it features a nontrivial energy gap and corner states, which is evidenced as a second-order topological insulator (SOTI). In addition, the band order for its low-energy bands switches at moderate tensile strain, during which topological phase transition from SOTI and topological semimetal to double Weyl semimetal (DWSM) happens, accompanied by the change in real Chern number formνR=1toνR=0. At the critical point for the phase transition, the system can be characterized as a 2D pseudospin-1 fermion. Beside Pd3(C6S6)2, we further identify the ferromagnetic monolayer Fe3(C6S6)2can also take the DWSM-to-SOTI phase transition, where the topological fermions and corresponding edge/corner states could be fully spin-polarized. This work has for the first time realized topological transition between conventional topological quantum state and a higher-order one in both nonmagnetic and magnetic MOFs.

Magnetic Real Chern Insulator in 2D Metal-Organic Frameworks.

Real Chern insulators have attracted great interest, but so far, their material realization is limited to nonmagnetic crystals and systems without spin-orbit coupling. Here, we reveal the magnetic real Chern insulator (MRCI) state in a recently synthesized metal-organic framework material Co3(HITP)2. Its ground state with in-plane ferromagnetic ordering hosts a nontrivial real Chern number, enabled by the C2zT symmetry and robustness against spin-orbit coupling. Distinct from previous nonmagnetic examples, the topological corner zero modes of MRCIs are spin-polarized. Furthermore, under small tensile strains, the material undergoes a topological phase transition from the MRCI to a magnetic double-Weyl semimetal phase, via a pseudospin-1 critical state. Similar physics can also be found in closely related materials Mn3(HITP)2 and Fe3(HITP)2, which also exist. Possible experimental detections and implications of an emerging magnetic flat band in the system are discussed.

Two-dimensional half-metallicity and fully spin-polarized topological fermions in monolayer EuOBr.

Two-dimensional (2D) half-metal and topological states have been the current research focus in condensed matter physics. Herein, we report a novel 2D material named EuOBr monolayer, which can simultaneously show 2D half-metal and topological fermions. This material shows a metallic state in the spin-up channel but a large insulating gap of 4.38 eV in the spin-down channel. In the conducting spin channel, the EuOBr monolayer shows the coexistence of Weyl points and nodal-lines near the Fermi level. These nodal-lines are classified by type-I, hybrid, closed, and open nodal-lines. The symmetry analysis suggests these nodal-lines are protected by the mirror symmetry, which cannot be broken even spin-orbit coupling is included because the ground magnetization direction in the material is out-of-plane [001]. The topological fermions in the EuOBr monolayer are fully spin-polarized, which can be meaningful for future applications in topological spintronic nano-devices.

Magnetic Electrides: High-Throughput Material Screening, Intriguing Properties, and Applications.

Electrides are a unique class of electron-rich materials where excess electrons are localized in interstitial lattice sites as anions, leading to a range of unique properties and applications. While hundreds of electrides have been discovered in recent years, magnetic electrides have received limited attention, with few investigations into their fundamental physics and practical applications. In this work, 51 magnetic electrides (12 antiferromagnetic, 13 ferromagnetic, and 26 interstitial-magnetic) were identified using high-throughput computational screening methods and the latest Materials Project database. Based on their compositions, these magnetic electrides can be classified as magnetic semiconductors, metals, or half-metals, each with unique topological states and excellent catalytic performance for N2 fixation due to their low work functions and excess electrons. The novel properties of magnetic electrides suggest potential applications in spintronics, topological electronics, electron emission, and as high-performance catalysts. This work marks the beginning of a new era in the identification, investigation, and practical applications of magnetic electrides.

A two-dimensional tunable double Weyl fermion in BL-α borophene.

Two-dimensional (2D) materials with nontrivial band crossings, namely linear or double Weyl points, have been attracting tremendous attention. However, it remains a challenge to find existing 2D materials that host such nontrivial states. Here, based on first-principles calculations and symmetry analysis, we discover that the recently synthesized BL-α borophene is a metal with a tunable double Weyl point. Remarkably, both bands forming the double Weyl point have upward band bending. In addition, it shows an anisotropic band dispersion when away from the double Weyl point. To characterize its anisotropy, we define a quantity G, which could be changed from 1 to infinity when going from the energy of the double Weyl point to the Fermi level. Furthermore, the outer band of the double Weyl point is sensitive to biaxial strain, and could be changed from upward bending to downward bending. During this process, it has a critical case, in which the outer-band becomes flat. The changes in outer-band induce a variation in the density of states around the double Weyl point, thus giving rise to changes in its macroscopic physical properties. Applying a uniaxial strain enables the double Weyl point to transform into a pair of Weyl points by breaking the threefold rotation of BL-α borophene. When breaking the inversion symmetry and in-plane twofold rotation symmetry by a vertical symmetry, the double Weyl point still persisted; meanwhile, an additional pair of linear Weyl points appears on the high-symmetry path, giving rise to a Weyl complex case. Overall, our work thus provides an existing 2D material, BL-α borophene, to study the nontrivial band crossings in 2D.

Multifold Fermions and Fermi Arcs Boosted Catalysis in Nanoporous Electride 12CaO·7Al2 O3.

Topological materials have been recently regarded as ideal catalysts for heterogeneous reactions due to their surface metallic states and high carrier mobility. However, the underlying relationship between their catalytic performance and topological states is under debate. It has been discovered that the electride 12CaO·7Al2 O3 (C12A7:4e- ) hosts multifold fermions and Fermi arcs on the (001) surface near the Fermi level due to the interstitial electrons. Through the comparison of catalytic performance under different doping and strain conditions, based on the hydrogen evolution process, it has been demonstrated that the excellent catalytic performance indeed originates from topological properties. A linear relationship between the length of Fermi arcs, and Gibbs free energy (ΔGH* ) has been found, which not only provides the direct evidence to link the enhanced catalytic performance and surface Fermi arc states, but also fully clarifies the fundamental mechanism in topological catalysis.

Movable triple points and Dirac points in centrosymmetric AB2 (A = Cr, Mo; B = Si, Ge) compounds.

Topological semimetals with nontrivial band crossing points have attracted widespread interest in recent years. Here, we propose that AB2 (A = Cr, Mo; B = Si, Ge) compounds are topological semimetals that feature a pair of triple points (TPs) on high-symmetry paths in the absence of spin-orbital coupling (SOC). In particular, the existence of this kind of TP is accompanied by a quadratic nodal line (QNL). In addition, we discover that these TPs are movable. Under a triaxial strain, we can change their positions on high-symmetry paths. When considering SOC, TPs transform into two pairs of type-II Dirac points along the high-symmetry path. Akin to TPs without SOC, each pair of Dirac points can also shift their positions on the high-symmetry paths under a triaxial strain. To characterize this property of TPs and Dirac points, we construct an effective model around the TPs and Dirac points, finding that there indeed exists a parameter that could characterize the movable properties for the TPs and Dirac points. According to the bulk-surface correspondence, we also discover that the length of the Fermi arcs that correspond to the nontrivial band crossings are also altered when changing their positions. Meanwhile, the shapes of Fermi arcs are also changed. Therefore, our work provides a platform to study the band crossings that are movable. The controllable fermions are beneficial to utilize the topological materials in nano-devices.

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.

Binary pentagonal auxetic materials for photocatalysis and energy storage with outstanding performances.

Since the discovery of penta-graphene, two-dimensional (2-D) pentagonal-structured materials have been highly expected to have desirable performance because of their unique structures and accompanied physical properties. Hence, based on the first-principles calculations, we performed a systematical study on the structure, stability, mechanical and electronic properties, and potential applications on carbon-based pentagonal materials with binary compositions, namely, Penta-CnX6-n (n = 1, 2, 4, 5; X = B, N, Al, Si, P, Ga, Ge, As). We found that eleven out of thirty-two Penta-CnX6-n have good stability and can be further studied. Among them, two materials, namely, Penta-C4P2 and Penta-C5P are metallic, and others are indirect band gap semiconductors, whose band gaps calculated by the HSE06 functional are in the range of 1.37-6.43 eV, covering the infrared-visible-ultraviolet regions. Furthermore, we found that metallic Penta-CnX6-n can become promising anode materials for Na-ion batteries (NIBs) with high storage capacity, while some semiconducting Penta-CnX6-n can become excellent water splitting photocatalysts. In addition, Penta-C4P2 and Penta-C2Al4 were found to have obvious in-plane negative Poisson's ratio (NPR) of -0.083 and -0.077, respectively. More interestingly, we found that Penta-C2Al4 exhibits a peculiar in-plane half negative Poisson's ratio (H-NPR) with the fundamental mechanism clarified. These outstanding performances endow binary pentagonal materials with excellent application prospects.

Two-dimensional metallic carbon allotrope with multiple rings for ion batteries.

Two-dimensional (2-D) materials, especially carbon allotropes, have larger storage capacity and faster diffusion rate due to their unique structures and are usually used in ion batteries. Recently, a new stable two-dimensional carbon allotrope, namely PAI-graphene, was reported by first-principles calculations. Due to its lightweight and multiple-ring structure, great stability and excellent properties, here, we theoretically reveal the excellent performance of PAI-graphene as an anode material for Li-/Na-ion batteries. Our results show that PAI-graphene has intrinsic metallicity before and after adsorption of Li/Na, which ensures that it has good conductivity when working as an electrode material. In addition, PAI-graphene exhibits quite low open circuit voltage (0.342-0.190 V for Li, 0.339-0.233 V for Na) and diffusion barrier (0.34 eV for Li, 0.17 eV for Na), which indicates its superiority as an anode material. Most noteworthily, the Na storage capacity of PAI-graphene is up to 1674 mA h g-1, which is much higher than that of most 2-D anode materials. Thus, we believe that PAI-graphene can be an outstanding anode material with outstanding performance.

Fully spin-polarized Weyl fermions and in/out-of-plane quantum anomalous Hall effects in a two-dimensional d0 ferromagnet.

The quantum anomalous Hall effect (QAHE) in intrinsic ferromagnets has attracted considerable attention recently. Previously, studies of the QAHE have mostly focused on the default assumption of out-of-plane magnetization. In fact, the QAHE can also be achieved via in-plane magnetization, but such candidate materials are very scarce. Here, we find that two-dimensional (2D) YN2 not only possesses the previously reported out-of-plane QAHE, but it also possesses a tunable in-plane QAHE. More importantly, unlike the previously reported in-plane QAHE in d/f-type ferromagnets, here we report the effect in a 2D d0 ferromagnet, namely YN2, for the first time. In the ground state, a YN2 monolayer has a half-metal band structure, and manifests six pairs of fully spin-polarized Weyl points at the Fermi level. When spin-orbit coupling is included, the YN2 monolayer can realize multiple topological phases, determined based on the magnetization direction. Under in-plane magnetization, the YN2 monolayer shows either the Weyl state or in-plane QAHE state. Remarkably, the Chern number (±1) and the propagating direction of QAHE edge channels can be continuously switched via shifting the direction of the in-plane magnetic field. When magnetization is applied out-of-plane, the YN2 monolayer realizes an out-of-plane QAHE phase with a high Chern number of 3. The nontrivial edge states for all the topological phases in the YN2 monolayer have been clearly identified. This work suggests that 2D YN2 is an excellent candidate for investigating in-plane QAHE phases in d0 ferromagnets.

Pentagonal B2C monolayer with extremely high theoretical capacity for Li-/Na-ion batteries.

Recently, two-dimensional (2-D) materials with a Penta-atomic-configuration such as Penta-graphene have received considerable attention because of their potential applications in electronics, spintronics and ion batteries. Previously, Penta-graphene has been proposed as an excellent anode material for Li-/Na-ion batteries with a high theoretical capacity (1489 mA h g-1). Here, based on the first-principles calculations, we report that a new 2-D material namely Penta-B2C can become another excellent anode material with even higher theoretical capacity for Li-/Na-ion batteries than Penta-graphene. Our results demonstrate that Li/Na atoms can be stably adsorbed on Penta-B2C. Meanwhile, Penta-B2C shows metallic conductivity during the adsorption. Most strikingly, the theoretical capacities of Penta-B2C are as high as 1594 for Li and 2391 mA h g-1 for Na, which are superior to those of the most known 2-D anode materials. Especially, the Na theoretical capacity of Penta-B2C sets a new record among known 2-D anode materials. In addition, Penta-B2C possesses relatively low open-circuit voltage and a low diffusion barrier for ions, which are vital for anode materials. These results highly promise that Penta-B2C can be an excellent anode material with a fast charge/discharge rate and extremely high theoretical capacity for Li-/Na-ion batteries.

Novel topological states of nodal points and nodal rings in 2D planar octagon TiB4.

Topological states of matter in two-dimensional (2D) materials have received increasing attention due to their potential applications in nanoscale spintronics. Here, we report the presence of unique topological electronic properties in a 2D planar octagon TiB4 compound. Particularly, without considering the spin-orbit coupling (SOC), we found that the material showed a coexistence of novel quadratic node (QN), and two different types of nodal rings (NRs), namely type-I and type-II. The protection mechanism of fermions has been fully clarified in this study. Furthermore, these fermions showed clear edge states. It is worth noting that QN had a topological charge of 2 since it is different from linear nodes and exhibit clear Fermi arc edge states. Under lattice strain, we found that the system could further exhibit rich topological phase transition. When SOC was included, we determined that these crossing points open very tiny energy gaps, which were smaller than previously reported 3D and 2D examples. These results show that monolayer TiB4 is an excellent nodal point and nodal ring semimetal, which also provides a feasible member for studying potential entanglements among multiple fermions.

HLB-MCX-Based Solid-Phase Extraction Combined with Liquid Chromatography-Tandem Mass Spectrometry for the Simultaneous Determination of Four Agricultural Antibiotics (Kasugamycin, Validamycin A, Ningnanmycin, and Polyoxin B) Residues in Plant-Origin Foods.

An ultrahigh-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method was established for the determination of four highly polar agricultural antibiotics kasugamycin, validamycin A, ningnanmycin, and polyoxin B in plant-derived foods. The samples were extracted with a 0.2% formic acid solution, purified by hydrophilic-lipophilic balance and mixed-mode cation-exchange solid-phase extraction, and then reconstituted for UPLC-MS/MS detection. The chromatographic analysis was performed on a BEH Amide column (100 mm × 2.1 mm, 1.7 µm) using gradient elution with a 0.1% formic acid solution and 0.1% formic acid acetonitrile as mobile phases. Method validation was performed on 15 matrices spiked at 0.02 (or 0.05), 0.5, and 2 mg/kg. The mean recovery rate ranged from 75 to 102% with relative standard deviations (RSD) was less than 20%. Good linearities (r > 0.99) in the range of 0.002-0.2 µg/mL were obtained. The limits of quantification (LOQs) were 0.02 and 0.05 mg/kg. Studies on the stability of the analytes in the stored kiwifruit samples showed that kasugamycin, validamycin A, and ningnanmycin were stable for at least 6 months, while polyoxin B was observed to be partially degraded (the degradation rate at 6 months was 31.3%). The method was demonstrated to be effective and reliable in real samples. In the kiwifruit samples treated after 7 days, no residues of ningnanmycin and polyoxin B were detected, while the residues of kasugamycin and validamycin A were 0.12 and 0.038 mg/kg, respectively.

Spin-Orbit Coupling-Determined Topological Phase: Topological Insulator and Quadratic Dirac Semimetals.

Our work reveals a class of three-dimensional materials whose main features are dominated by d-orbital states. Their unique properties are derived from the low-energy states t2g. Without spin-orbital coupling (SOC), we find a triple degenerate point with a quadratic dispersion, demonstrated by an effective Hamiltonian. When SOC is included, the sign of SOC could determine the topological phases of materials: a negative SOC contributes a Dirac semimetal phase with a quadratic energy dispersion, whereas a positive SOC leads to a strong topological insulator phase. There exist clear surface states for the corresponding topological phases. Very interestingly, by application of a triaxial strain, the sequence of bands can be exchanged, as do the topological phases. In particular, there exists a 6-fold degenerate point under a critical strain. Furthermore, we use a uniaxial compressive/tensile strain, changing the quadratic Dirac point into a linear Dirac/strong topological insulator phase.

Weyl Fermions in VI3 Monolayer.

We report the presence of a Weyl fermion in VI3 monolayer. The material shows a sandwich-like hexagonal structure and stable phonon spectrum. It has a half-metal band structure, where only the bands in one spin channel cross the Fermi level. There are three pairs of Weyl points slightly below the Fermi level in spin-up channel. The Weyl points show a clean band structure and are characterized by clear Fermi arcs edge state. The effects of spin-orbit coupling, electron correlation, and lattice strain on the electronic band structure were investigated. We find that the half-metallicity and Weyl points are robust against these perturbations. Our work suggests VI3 monolayer is an excellent Weyl half-metal.

Magnetism, half-metallicity, and topological signatures in Fe2-xVxPO5 (x = 0, 0.5, 1, 1.5, 2) materials: a potential class of advanced spintronic materials.

Novel spintronic materials combining both magnetism and nontrivial topological electronic structures have attracted increasing attention recently. Here, we systematically studied the doping effects, magnetism, half-metallicity, and topological properties in the family of Fe2-xVxPO5 (x = 0, 0.5, 1, 1.5, 2) compounds. Our results show that Fe2PO5 takes an antiferromagnetic (AFM) ordering with a zero total magnetic moment. Meanwhile, the material hosts a Dirac nodal line and a Weyl nodal line near the Fermi level. V2PO5 is a ferromagnetic (FM) nodal line half-metal with a 100% spin-polarized Weyl nodal line. After doping, we find that Fe1.5V0.5PO5, Fe1V1PO5 and Fe0.5V1.5PO5 all take ferrimagnetic (FiM) ordering, with the Fe and V atoms taking opposite spin directions. Both Fe1.5V0.5PO5 and Fe0.5V1.5PO5 are FiM half-metals. Meanwhile, they show several pairs of fully spin-polarized Weyl points near the Fermi level. Fe1V1PO5 is a FiM semiconductor with different sizes of band gaps in different spin channels. These Fe2-xVxPO5 materials not only provide a good research platform to study the novel properties combining magnetism and nontrivial band topology, but also have promising applications in spintronic applications.

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

Nonsymmorphic symmetry has been proved to protect band crossings in topological semimetals/metals. In this work, based on the symmetry analysis and first-principles calculations, we reveal rich topological phases in compounds Pd4X (X = S, Se), which are protected by nonsymmorphic symmetry. In the absence of spin-orbit coupling (SOC), it shows the coexistence of the type-I Weyl point and type-II Weyl point. Here, due to the screw rotation, the type-I Weyl point takes an hourglass form. However, this hourglass Weyl point can be gapped in the presence of SOC. Furthermore, a combination of nonsymmorphic twofold screw-rotational symmetry and time-reversal symmetry protects a nodal surface. Particularly, this nodal surface is robust against SOC. In addition, a combination of the glide mirror and time-reversal symmetry contributes a nodal line of double degeneracy. In the presence of SOC, there emerges hybridization of type-I and type-II Weyl points. Meanwhile, there also appears a Dirac nodal line-a fourfold degenerate nodal line under SOC, which is protected by nonsymmorphic symmetries. Our works suggest realistic materials to study Weyl nodes of type-I and type-II, and their hybridization, as well as symmetry-protected nodal surfaces and Dirac nodal lines.