Chern Insulator and Chern Half-Metal States in the Two-Dimensional Spin-Gapless Semiconductor Mn2C6S12

Quintuple Function Integration in Two-Dimensional Cr(II) Five-Membered Heterocyclic Metal Organic Frameworks via Tuning Ligand Spin and Lattice Symmetry

Two-dimensional (2D) semiconductors (SCs) integrated with two or more functions are the cornerstone for constructing multifunctional nanodevices but remain largely limited. Here, by tuning the spin state of organic linkers and the symmetry/topology of crystal lattices, we predict a class of unprecedented multifunctional SCs in 2D Cr(II) five-membered heterocyclic metal organic frameworks that simultaneously possess auxetic effect, room-temperature ferrimagnetism, chiral ferroelectricity (FE), electrically reversible spin polarization, and topological nodal lines/points. Taking 2D Cr(TDZ)₂ (TDZ = 1.2.5-thiadiazole) as an exemplification, the auxetic effect is produced by the antitetra-chiral lattice structure. The high temperature ferrimagnetism originates from the strong d-p direct magnetic exchange interaction between Cr cations and TDZ doublet radical anions. Meanwhile, the clockwise-counterclockwise alignment of TDZ's dipoles results in unique 2D chiral FE with atomic-scale vortex-antivortex states. 2D Cr(TDZ)₂ is an intrinsic bipolar magnetic SC where half-metallic conduction with switchable spin-polarization direction can be induced by applying a gate voltage. In addition, the symmetry of the little group C₄ of the lattice structure endows 2D Cr(TDZ)₂ with topological nodal lines and a quadratic nodal point in the Brillouin zone near the Fermi level.

Intrinsic Second-Order Topological Insulator in Two-Dimensional Covalent Organic Frameworks

As an intriguing topological phase, higher-order topological insulators have attracted tremendous attention, but the candidate materials are limited in artificial and inorganic systems. In this work, we propose a universal approach to search for two-dimensional (2D) second-order topological insulators (SOTIs) in covalent organic frameworks (COFs) with C₃ symmetric cores. The underlying mechanism is illustrated through tight-binding calculations in a star lattice, showing the 2D SOTI in an overlooked energy window between two Kagome-bands with four types of nontrivial band structures. The emergence of the unique topological edge and corner states can be understood from the Su-Schrieffer-Heeger model. Furthermore, using the frontier orbital of the monomer building block as an indicator, the 2D SOTI is directly confirmed in three realistic COFs by first-principles calculations. Our results not only extend the concept of organic topological insulators from first-order to second-order but also demonstrate the universal existence of intrinsic higher-order topology in 2D COFs.

Mini-review of interesting properties in Mn2CoAl bulk and films

Heusler compounds exhibit many interesting properties, such as high thermopower, magnetocaloric properties, and even topological insulator states. Heusler Mn₂CoAl alloy has been experimentally and theoretically proposed as a promising spin-gapless semiconductor with novel electronic, magnetic, spintronic, transport, and topological properties. Furthermore, the spin-gapless semiconducting-like behaviors are also predicted in Mn₂CoAl films by measuring the transport and magnetic properties. This mini-review systematically summarizes the interesting properties of Mn₂CoAl bulk and Mn₂CoAl-based films. This mini-review is hoped to guide further experimental investigations and applications in the particular scientific community.

Two-dimensional spin-gapless semiconductors: A mini-review

In the past decade, two-dimensional (2D) materials and spintronic materials have been rapidly developing in recent years. 2D spin-gapless semiconductors (SGSs) are a novel class of ferromagnetic 2D spintronic materials with possible high Curie temperature, 100% spin-polarization, possible one-dimensional or zero-dimensional topological signatures, and other exciting spin transport properties. In this mini-review, we summarize a series of ideal 2D SGSs in the last 3 years, including 2D oxalate-based metal-organic frameworks, 2D single-layer Fe₂I_2, 2D Cr₂X₃ (X = S, Se, and Te) monolayer with the honeycomb kagome (HK) lattice, 2D CrGa₂Se₄ monolayer, 2D HK Mn–cyanogen lattice, 2D MnNF monolayer, and 2D Fe₄N₂ pentagon crystal. The mini-review also discusses the unique magnetic, electronic, topological, and spin-transport properties and the possible application of these 2D SGSs. The mini-review can be regarded as an improved understanding of the current state of 2D SGSs in recent 3 years.

Spin-Gapless States in Two-Dimensional Molecular Ferromagnet Fe2(TCNQ)2

Two-dimensional van der Waals magnetic atomic crystals have provided unprecedented access to magnetic ground states due to a quantum confinement effect. Here, using first-principles calculations, we demonstrate a spin-gapless molecular ferromagnet, namely, Fe₂(TCNQ)₂, with superior mechanical stability and a remarkable linear Dirac cone, which can be exfoliated from its already-synthesized van der Waals crystal. Especially, Young's modulus has values of 175.28 GPa·nm along the x- and y-directions with a Poisson's ratio of 0.29, while the Curie temperature within the Ising model is considerably higher than room temperature. Furthermore, spin-orbit coupling can open a band gap at the Dirac point, leading to topologically nontrivial electronic states characterized by an integer value of the Chern number and the edge states of its nanoribbon. Our results offer versatile platforms for achieving plastic spin filtering or a quantum anomalous Hall effect with promising applications in spintronics devices.

Spin-Gapless Semiconductors

The spin-gapless semiconductors (SGSs) are a new class of zero-gap materials which have fully spin polarized electrons and holes. They bridge the zero-gap materials and the half-metals. The band structures of the SGSs can have two types of energy dispersion: Dirac linear dispersion and parabolic dispersion. The Dirac-type SGSs exhibit fully spin polarized Dirac cones, and offer a platform for massless and fully spin polarized spintronics as well as dissipationless edge states via the quantum anomalous Hall effect. With fascinating spin and charge states, they hold great potential for spintronics. There have been tremendous efforts worldwide to find suitable candidates for SGSs. In particular, there is an increasing interest in searching for Dirac type SGSs. In the past decade, a large number of Dirac or parabolic type SGSs have been predicted by density functional theory, and some parabolic SGSs have been experimentally demonstrated. The SGSs hold great potential for spintronics, electronics, and optoelectronics with high speed and low-energy consumption. Here, both the Dirac and the parabolic types of SGSs in different material systems are reviewed and the concepts of the SGS, novel spin and charge states, and the potential applications of SGSs in next-generation spintronic devices are outlined.

Electronic Structure Modeling of Metal-Organic Frameworks

Owing to their molecular building blocks, yet highly crystalline nature, metal-organic frameworks (MOFs) sit at the interface between molecule and material. Their diverse structures and compositions enable them to be useful materials as catalysts in heterogeneous reactions, electrical conductors in energy storage and transfer applications, chromophores in photoenabled chemical transformations, and beyond. In all cases, density functional theory (DFT) and higher-level methods for electronic structure determination provide valuable quantitative information about the electronic properties that underpin the functions of these frameworks. However, there are only two general modeling approaches in conventional electronic structure software packages: those that treat materials as extended, periodic solids, and those that treat materials as discrete molecules. Each approach has features and benefits; both have been widely employed to understand the emergent chemistry that arises from the formation of the metal-organic interface. This Review canvases these approaches to date, with emphasis placed on the application of electronic structure theory to explore reactivity and electron transfer using periodic, molecular, and embedded models. This includes (i) computational chemistry considerations such as how functional, k-grid, and other model variables are selected to enable insights into MOF properties, (ii) extended solid models that treat MOFs as materials rather than molecules, (iii) the mechanics of cluster extraction and subsequent chemistry enabled by these molecular models, (iv) catalytic studies using both solids and clusters thereof, and (v) embedded, mixed-method approaches, which simulate a fraction of the material using one level of theory and the remainder of the material using another dissimilar theoretical implementation.

2D CrCl2(pyrazine)2 monolayer: high-temperature ferromagnetism and half-metallicity

The ferromagnetism in Cr-based monolayers is of current interest (2019 Nat. Nanotechnol. 14 408), however, the Curie temperature is low. How can we enhance the thermal stability of ferromagnetism? Motivated by the recent synthesis of the layered conductive magnet CrCl₂(pyrazine)₂ (2018 Nat. Chem. 10 1056), we perform first-principles calculations and Monte Carlo simulations to demonstrate that the exfoliated 2D CrCl₂(pyrazine)₂ monolayer is stable dynamically and thermally, and it is a ferromagnetic half-metal with a sizeable band gap of 2.8 eV in the semiconducting channel, and the strong in-plane Cr-Cr interaction results in a large magnetic anisotropy energy. Moreover, the sheet exhibits a high Curie temperature of 350 K due to the enhanced magnetic exchange interaction resulting from the aromatic property of pyrazine. All of these intriguing features endow 2D CrCl₂(pyrazine)₂ sheet with good potentials for applications in nanoscale spintronics devices.

Highly spin-polarized electronic structure and magnetic properties of Mn2.25Co0.75Al1−xGex Heusler alloys: first-principles calculations

Highly spin-polarized half-metals (HMs) and spin-gapless semiconductors (SGSs) are the promising candidates in spintronic devices. However, the HM and SGS Heusler materials are very sensitive to the stoichiometric defects and lattice distortion, which will be not beneficial to the practical applications. Here, the electronic structure and magnetic properties of Mn_2.25Co_0.75Al_1−xGe_x (x = 0, 0.25, 0.50, 0.75 and 1.00) Heusler alloys were investigated by first-principles calculations. Large negative formation energy, cohesive energy and phonon spectra confirm that the Mn_2.25Co_0.75Al_1−xGe_x alloys are stable. It is found that Mn_2.25Co_0.75Al_1−xGe_x with x = 0, 0.25, 0.75 and 1.00 show robust ferrimagnetic HM characteristics, while Mn_2.25Co_0.75Al_0.5Ge_0.5 shows robust SGS characteristic. Under the hydrostatic and uniaxial strains, Mn_2.25Co_0.75Al_1−xGe_x exhibit a series of rich electronic transitions. The magnetic anisotropy of Mn_2.25Co_0.75Al_1−xGe_x turns from the in-plane [100] direction to the out-of-plane [001] one by applying the uniaxial strains. The results suggest that the complete spin polarizations of Mn_2.25Co_0.75Al_1−xGe_x alloys are robust against the stoichiometric defects and lattice distortion, which have potential applications in spintronic devices.

The complete spin polarizations of Mn_2.25Co_0.75Al_1−xGe_x are proved to be robust against stoichiometric defect and lattice deformation, whose easy magnetization direction can be manipulated from in-plane direction to out-of-plane one under uniaxial strain.

Serendipity of a topological nontrivial band gap in the 2D borophene subunit lattice with broken mirror symmetry

The exotic electronic band structures featured by Dirac cones and topological phases in two-dimensional (2D) materials are regarded as the holy grail of the next-generation electronic devices. Here we propose a 2D tungsten boride (WB4) lattice to concurrently host these interesting properties. Based on first-principles calculations, we demonstrate that in the absence of spin-orbit coupling (SOC), the mirror symmetry protects the WB4 lattice to spawn multiple Dirac bands around the Fermi level with high velocities. However, the broken mirror symmetry induces one cone to be opened with a small band gap, and gives rise to a nontrivially topological phase characterized by the non-zero Z2 topological invariant. Interestingly, topologically nontrivial states of the lattice without mirror symmetry are robust within external biaxial tension, which is confirmed from the appearance of gapless edge states in their nanoribbon structure. Our results provide a versatile platform for hosting nontrivial topological states usable for important nanoelectronic device applications.

Palladium (III) Fluoride Bulk and PdF3/Ga2O3/PdF3 Magnetic Tunnel Junction: Multiple Spin-Gapless Semiconducting, Perfect Spin Filtering, and High Tunnel Magnetoresistance

Spin-gapless semiconductors (SGSs) with Dirac-like band crossings may exhibit massless fermions and dissipationless transport properties. In this study, by applying the density functional theory, novel multiple linear-type spin-gapless semiconducting band structures were found in a synthesized R3−c-type bulk PdF₃ compound, which has potential applications in ultra-fast and ultra-low power spintronic devices. The effects of spin-orbit coupling and on-site Coulomb interaction were determined for the bulk material in this study. To explore the potential applications in spintronic devices, we also performed first-principles combined with the non-equilibrium Green’s function for the PdF₃/Ga₂O₃/PdF₃ magnetic tunnel junction (MTJ). The results suggested that this MTJ exhibits perfect spin filtering and high tunnel magnetoresistance (~5.04 × 10⁷).

High spin polarization in formamidinium transition metal iodides: first principles prediction of novel half-metals and spin gapless semiconductors

The electronic structure and magnetic properties of ten formamidinium transition metal iodides in the ground state and under strain have been studied. These formamidinium transition metal iodides have a stable cubic perovskite structure. In the ground state, FAVI3 is a spin gapless semiconductor, and FAScI3, FATiI3, FACrI3, FAFeI3, FACoI3 and FANiI3 are ferromagnetic half-metals. They all have 100% spin polarization and integer total magnetic moment. Under the action of strain, the high spin polarization of some formamidinium transition metal iodides can still be well maintained, and several novel spin gapless semiconductors such as FATiI3, FAFeI3 and FACoI3 have been discovered. Magnetic studies show that these formamidinium transition metal iodides with half-metal, semiconductor and spin-gapless semiconductor properties have integral total magnetic moments under strain ranging from -10.0% to 10.0%. These newly discovered half-metallic ferromagnetic materials and spin gapless semiconductors have broad application prospects in the field of spintronics due to their high spin polarization.

Transition-Metal Dihydride Monolayers: A New Family of Two-Dimensional Ferromagnetic Materials with Intrinsic Room-Temperature Half-Metallicity

Two-dimensional (2D) ferromagnetic materials with intrinsic half-metallicity are highly desirable for nanoscale spintronic applications. Here, we predict a new and stable family of 2D transition-metal dihydride (MH₂; M = Sc, Ti, V, Cr, Fe, Co, Ni) monolayers with novel properties. Our density functional theory computation shows that CoH₂ and ScH₂ monolayers are ferromagnetic metals, while the others are antiferromagnetic semiconductors. In particular, the CoH₂ monolayer is a perfect half-metal with a wide spin gap of 3.48 eV. The ScH₂ monolayer can also possess half-metallicity through hole doping. Most importantly, our Monte Carlo simulations show that the CoH₂ monolayer possesses an above-room-temperature Curie point (339 K), while that of the ScH₂ monolayer can also reach 160 K. A synthetic approach is proposed to realize CoH₂ and ScH₂ monolayers in the laboratory. Notably, their half-metallicity can be well maintained on substrates. The new family of MH₂ monolayers are promising functional materials for spintronic applications due to their novel magnetic properties.

Kane Fermion in a Two-Dimensional π-Conjugated Bis(iminothiolato)nickel Monolayer

Massless Kane fermions revealed in zinc-blende semiconductors have recently gained interest in the broad study of relativistic materials. In particular, two-dimensional (2D) Kane fermions were expected to be hybrids of pseudospin-1 and -1/2 Dirac fermions. Based on first-principles calculations, we demonstrated that 2D Kane fermions can be realized in a recently synthesized metal-organic framework, namely, bis(iminothiolato)nickel monolayer. A slight compression takes the system from a semimetal to a semiconductor. At the critical strain of ∼1%, the upper and lower conical bands linearize and touch at a single point intersecting a flat band, showing the same dispersion as the pseudospin-1 Dirac-Weyl systems. We adopted a tight-binding Hamiltonian of a line-centered honeycomb lattice to reveal the origins and topology of the electronic band structure. The coexistence of Kane-type and Dirac-type spectra in the bis(iminothiolato)nickel monolayer is expected to benefit the study of multi quasiparticle effects.

Zr2Si: an antiferromagnetic Dirac MXene

The antiferromagnetic ground state of Zr₂Si MXene was determined to exhibit anisotropic Dirac cones with Fermi velocities comparable to that in graphene.