Novel valley character and tunable quasi-half-valley metal state in Janus monolayer VSiGeP4.

The manipulation and regulation of valley characteristics have aroused widespread interest in emerging information fields and fundamental research. Realizing valley polarization is one crucial issue for spintronic and valleytronic applications, the concepts of a half-valley metal (HVM) and ferrovalley (FV) materials have been put forward. Then, to separate electron and hole carriers, a fresh concept of a quasi-HVM (QHVM) has been proposed, in which only one type of carrier is valley polarized for electron and hole carriers. Based on first-principles calculations, we demonstrate that the Janus monolayer VSiGeP4 has QHVM character. To well regulate the QHVM state, strain engineering is utilized to adjust the electronic and valley traits of monolayer VSiGeP4. In the discussed strain range, monolayer VSiGeP4 always favors the ferromagnetic ground state and out-of-plane magnetization, which ensures the appearance of spontaneous valley polarization. It is found that the QHVM state can be induced in different electronic correlations (U), and the strain can effectively tune the valley, magnetic, and electronic features to maintain the QHVM state under various U values. Our work opens up a new research idea in the design of multifunctional spintronic and valleytronic devices.

Electronic-correlation induced sign-reversible Berry phase and quantum anomalous valley Hall effects in Janus monolayer OsClBr.

Topological phase transition can be induced by electronic correlation effects combined with spin-orbit coupling (SOC). Here, based on the first-principles calculations +U approach, the influence of electronic correlation effects and SOC on topological and electronic properties of the Janus monolayer OsClBr is investigated. With intrinsic out-of-plane (OOP) magnetic anisotropy, the Janus monolayer OsClBr exhibits a sequence of states, namely, the ferrovalley (FV) to half-valley-metal (HVM) to quantum anomalous valley Hall effect (QAVHE) to HVM to FV states with increasing U values. The QAVHE is characterized by a chiral edge state linking the conduction and valence bands with a Chern number C = 1, which is closely associated with the band inversion between dx2-y2/dxy and dz2 orbitals, and sign-reversible Berry curvature. The section with larger U values (2.31-2.35 eV) is very essential for determining the new HVM and QAVHE states, and also proves that a strong electron correlation effect exists in the interior of the Janus monolayer OsClBr. When taking into consideration a representative U value (U = 2.5 eV), a valley polarization value of 157 meV can be observed, which can be switched by reversing the magnetization direction of Os atoms. It is noteworthy that the Curie temperature (TC) strongly depends on the electronic correlation effects. Our work provides a comprehensive discussion on the electronic and topological properties of the Janus monolayer OsClBr, and demonstrates that the electronic correlation effects combined with SOC can drive the emergence of QAVHE, which will open up new opportunities for valleytronic, spintronic, and topological nanoelectronic applications.

High excellent hydrogen evolution characteristics and novel mechanism of two-dimensional tetra-phase OsN2 and ReN2.

It is essential to find a kind of electrocatalyst for hydrogen evolution reduction (HER) comparable with a noble metal that has good conductivity and abundant active sites. Based on systematic searches by first-principles calculations, we discovered two-dimensional transition-metal nitrides, tetra-phase OsN2 and ReN2 monolayers, as potential HER electrocatalysts with superior thermodynamic and kinetic stability. They exhibited excellent catalytic activity due to the presence of multiple active sites with a density of 8 × 1015 site per cm2 and an overpotential close to 0. In addition, we also found that the synergistic effect of strain and coverage makes them have a good hydrogen evolution activity. The ΔGH of the OsN2 monolayer at 1% tensile strain under 3/4 hydrogen coverage is 0.02 eV, and that of ReN2 at 1/2 hydrogen coverage could decrease to 0.001 eV. Different from other common transition metal nitrides, we found that the active sites of OsN2 and ReN2 monolayers are both at nitrogen atoms, which could be further understood by the crystal orbital Hamiltonian population analysis between N and metal atoms. All these interesting findings not only provide new excellent candidates but also provide new insights into the mechanism of hydrogen evolution of nitrides.

Two-dimensional Weyl semi-half-metallic NiCS3 with a band structure controllable by the direction of magnetization.

Two-dimensional (2D) Weyl semi-half-metals (WSHMs) have attracted tremendous interest for their fascinating properties combining half-metallic ferromagnetism and Weyl fermions. In this work, we present a NiCS3 monolayer as a new 2D WSHM material using systematic first-principles calculations. It has 12 fully spin-polarized Weyl nodal points in one spin channel with a Fermi velocity of 3.18 × 105 m s-1 and a fully gapped band structure in the other spin channel. It exhibits good mechanical and thermodynamic stabilities and the Curie temperature is estimated to be 403 K. The Weyl points are protected by vertical mirror plane symmetry along Γ-K, and each of them remains gapless even under spin-orbit coupling when the direction of spin is perpendicular to the Γ-K line including the Weyl point, which makes it possible to control the opening and closing of Weyl points by applying and rotating external magnetic fields. Our work not only provides a promising 2D WSHM material to explore the fundamental physics of symmetry protected ferromagnetic Weyl fermions, but also reveals a potential mechanism of band engineering of 2D WSHM materials in spintronics.

Discovery of a ferroelastic topological insulator in a two-dimensional tetragonal lattice.

Ferroelasticity and band topology are two intriguing yet distinct quantum states of condensed matter materials. Their coexistence in a single two-dimensional (2D) lattice, however, has never been observed. Here, we found that the 2D tetragonal HfC monolayer allowed simultaneous presence of ferroelastic and topological orders. By using first-principles calculations, we found that it could allow a low switching barrier with reversible strain of 17.4%, indicating that the anisotropic properties are achievable experimentally for a 2D tetragonal lattice. More interestingly, the tuning of topological behaviors with strain led to spin-separated and gapless edge states, that is, the quantum spin Hall effect. These findings from the coupling of two quantum orders offer insights into ferroelastic control over topological edge states for achieving multifunctional properties in next-generation 2D nanodevices.

Prediction of topological crystalline insulators and topological phase transitions in two-dimensional PbTe films.

Topological phases, especially topological crystalline insulators (TCIs), have been intensively explored and observed experimentally in three-dimensional (3D) materials. However, two-dimensional (2D) films are explored much less than 3D TCIs, and even 2D topological insulators. Based on ab initio calculations, here we investigate the electronic and topological properties of 2D PbTe(001) few-layer films. The monolayer and trilayer PbTe are both intrinsic 2D TCIs with a large band gap reaching 0.27 eV, indicating a high possibility for room-temperature observation of quantized conductance. The origin of the TCI phase can be attributed to the px,y-pz band inversion, which is determined by the competition of orbital hybridization and the quantum confinement effect. We also observe a semimetal-TCI-normal insulator transition under biaxial strains, whereas a uniaxial strain leads to Z2 nontrivial states. In particular, the TCI phase of a PbTe monolayer remains when epitaxially grown on a NaI semiconductor substrate. Our findings on the controllable quantum states with sizable band gaps present an ideal platform for realizing future topological quantum devices with ultralow dissipation.

Controllable electronic and magnetic properties in a two-dimensional germanene heterostructure.

The control of spin without a magnetic field is one of the challenges in developing spintronic devices. Here, based on first-principles calculations, we predict a new kind of ferromagnetic half-metal (HM) with a Curie temperature of 244 K in a two-dimensional (2D) germanene van der Waals heterostructure (HTS). Its electronic band structures and magnetic properties can be tuned with respect to external strain and electric field. More interestingly, a transition from HM to bipolar-magnetic-semiconductor (BMS) to spin-gapless-semiconductor (SGS) in a HTS can be realized by adjusting the interlayer spacing. These findings provide a promising platform for 2D germanene materials, which hold great potential for application in nanoelectronic and spintronic devices.

Tunable electronic properties induced by a defect-substrate in graphene/BC3 heterobilayers.

We perform first-principles calculations to study the geometric, energetics and electronic properties of graphene supported on BC3 monolayer. The results show that overall graphene interacts weakly with BC3 monolayer via van der Waals interaction. The energy gap of graphene can be up to ∼0.162 eV in graphene/BC3 heterobilayers (G/BC3 HBLs), which is large enough for the gap opening at room temperature. We also find that the interlayer spacing and in-plane strain can tune the band gap of G/BC3 HBLs effectively. Interestingly, the characteristics of a Dirac cone with a nearly linear band dispersion relationship of graphene can be preserved, accompanied by a small electron effective mass, and thus the higher carrier mobility is still expected. These findings provide a possible way to design effective FETs out of graphene on a BC3 substrate.

Tunable electronic and magnetic properties in germanene by alkali, alkaline-earth, group III and 3d transition metal atom adsorption.

We performed first-principles calculations to study the adsorption characteristics of alkali, alkali-earth, group III, and 3d transition-metal (TM) adatoms on germanene. We find that the adsorption of alkali or alkali-earth adatoms on germanene has minimal effects on geometry of germanene. The significant charge transfer from alkali adatoms to germanene leads to metallization of germanene, whereas alkali-earth adatom adsorption, whose interaction is a mixture of ionic and covalent, results in semiconducting behavior with an energy gap of 17-29 meV. For group III adatoms, they also bind germanene with mixed covalent and ionic bonding character. Adsorption characteristics of the transition metals (TMs) are rather complicated, though all TM adsorptions on germanene exhibit strong covalent bonding with germanene. The main contributions to the strong bonding are from the hybridization between the TM 3d and Ge pz orbitals. Depending on the induced-TM type, the adsorbed systems can exhibit metallic, half-metallic, or semiconducting behavior. Also, the variation trends of the dipole moment and work function with the adsorption energy across the different adatoms are discussed. These findings may provide a potential avenue to design new germanene-based devices in nanoelectronics.

Tunable abundant valley Hall effect and chiral spin-valley locking in Janus monolayer VCGeN4.

It is both conceptually and practically fascinating to explore fundamental research studies and practical applications of two-dimensional systems with the tunable abundant valley Hall effect. In this work, based on first-principles calculations, the tunable abundant valley Hall effect is proved to appear in Janus monolayer VCGeN4. When the magnetization is along the out-of-plane direction, VCGeN4 is an intrinsic ferromagnetic semiconductor with a valley feature. The intriguing spontaneous valley polarization exists in VCGeN4 due to the common influence of broken inversion and time-reversal symmetries, which makes it easier to realize the anomalous valley Hall effect. Furthermore, we observe that the valley-non-equilibrium quantum anomalous Hall effect is driven by external strain, which is located between two half-valley-metal states. When reversing the magnetization, the spin flipping makes the position of the edge state to change from one valley to another valley, demonstrating an intriguing behavior known as chiral spin-valley locking. Although the easy magnetic axis orientation is along the in-plane direction, we can utilize an external magnetic field to transform the magnetic axis orientation. Moreover, it is found that the valley state, electronic and magnetic properties can be well regulated by the electric field. Our works explore the mechanism of the tunable abundant valley Hall effect by applying an external strain and electric field, which provides a perfect platform to investigate the spin, valley, and topology.

Spontaneous valley polarization and valley-nonequilibrium quantum anomalous Hall effect in Janus monolayer ScBrI.

Topology and ferrovalley (FV) are two essential concepts in emerging device applications and the fundamental research field. To date, relevant reports are extremely rare about the coupling of FV and topology in a single system. By Monte Carlo (MC) simulations and first-principles calculations, a stable intrinsic FV ScBrI semiconductor with high Curie temperature (TC) is predicted. Because of the combination of spin-orbital coupling (SOC) and exchange interaction, the Janus monolayer ScBrI shows a spontaneous valley polarization of 90 meV, which is located in the top valence band. For the magnetization direction perpendicular to the plane, the changes from FV to half-valley-metal (HVM), to valley-nonequilibrium quantum anomalous Hall effect (VQAHE), to HVM, and to FV can be induced by strain engineering. It is worth noting that there are no particular valley polarization and VQAHE states for in-plane (IP) magnetic anisotropy. By obtaining the real magnetic anisotropy energy (MAE) under different strains, due to spontaneous valley polarization, intrinsic out-of-plane (OOP) magnetic anisotropy, a chiral edge state, and a unit Chern number, the VQAHE can reliably appear between two HVM states. The increasing strains can induce VQAHE, which can be clarified by a band inversion between dx2-y2/dxy and dz2 orbitals, and a sign-reversible Berry curvature. Once synthesized, the Janus monolayer ScBrI would find more significant applications in topological electronic, valleytronic, and spintronic nanodevices.

Two -dimensional semimetal AlSb monolayer with multiple nodal-loops and extraordinary transport properties under uniaxial strain.

Two-dimensional (2D) nodal-loop semimetal (NLSM) materials have attracted much attention for their high-speed and low-consumption transporting properties as well as their fantastic symmetry protection mechanisms. In this paper, using systematic first-principles calculations, we present an excellent NLSM candidate, a 2D AlSb monolayer, in which the conduction and valence bands cross with each other forming fascinating multiple nodal-loop (NL) states. The NLSM properties of the AlSb monolayer are protected by its glide mirror symmetry, which was confirmed using a symmetry-constrained six-band tight-binding model. The transport properties of the AlSb monolayer under in-plane uniaxial strains are also studied, based on a non-equilibrium Green's function method. It is found that both compressive and tensile strains from -10% to 10% improve the transporting properties of AlSb, and it is interesting to see that flexure configurations are energetically favored when compressive uniaxial strains are applied. Our studies not only provide a novel 2D NLSM candidate with a new symmetry protection mechanism, but also raise the novel possibility for the detection of out-of-plane flexure in 2D semimetal materials.

Anisotropic nodal loop in NiB2 monolayer with nonsymmorphic configuration.

Two-dimensional (2D) materials featuring a nodal-loop (NL) state have been drawing considerable attention in condensed matter physics and materials science. Owing to their structural polymorphism, recent high-profile metal-boride films have great advantages and the potential to realize a NL. Herein, a 2D NiB2 monolayer with an anisotropic NL nature is proposed and investigated using first-principles calculations. We show that the NiB2 monolayer has excellent thermal dynamics stability, suggesting the possibility of its synthesis in experiments. Remarkably, the NL with a considerable Fermi velocity is demonstrated to be protected by nonsymmorphic glide mirror symmetry, instead of the widely known horizontal mirror symmetry. Accompanied by the proper preservation of the NL, strain engineering can not only regulate the anisotropy of the NL but also give rise to a self-doping phenomenon characterized by effective modulation of the carrier type and concentration. Moreover, this NL state is robust against the correlation effect. These findings pave the way for exploring nonsymmorphic-symmetry-enabled NL nature in 2D metal-borides.

Glide Mirror Plane Protected Nodal-Loop in an Anisotropic Half-Metallic MnNF Monolayer.

Two-dimensional (2D) nodal-loop (NL) semimetals have attracted tremendous attention for their abundant physics and potential device applications, whereas the realization of gapless NL semimetals robust against spin-orbit coupling (SOC) remains a big challenge. Recently, breakthroughs have been made with the realization of gapless NL semimetals in 2D half-metallic materials, where NLs were protected by a horizontal mirror plane symmetry. Here we first propose an alternative nonsymmorphic horizontal glide mirror plane symmetry which could protect the NLs in 2D materials. On the basis of comprehensive first-principles calculations and symmetry analysis, we found that the glide mirror symmetry together with intrinsic out-of-plane spin polarization can protect the NL against SOC in a half-metallic semimetal, namely, the MnNF monolayer. Moreover, we predict that the MnNF monolayer has strong anisotropic characteristics, tunable band structure by changing the magnetization direction, and 100% spin-polarized transport properties. Our work not only provides a novel 2D half-metallic semimetal with strong anisotropy but also broadens the scope of 2D nodal-loop materials.

Prediction of high-temperature Chern insulator with half-metallic edge states in asymmetry-functionalized stanene.

A great obstacle for the practical applications of the quantum anomalous Hall (QAH) effect is the lack of suitable two-dimensional (2D) materials with a sizable nontrivial band gap, high Curie temperature, and high carrier mobility. Based on first-principles calculations, here, we propose the realizations of these intriguing properties in asymmetry-functionalized 2D SnHN and SnOH lattices. Spin-polarized band structures reveal that SnOH monolayer exhibits a spin gapless semiconductor (SGS) feature, whereas SnNH is converted to SGS under compressive strain. The Curie temperature of SnOH reaches 266 K, as predicted by Monte Carlo simulation, and it is comparable to the room temperature. When the spin and orbital degrees of freedom are allowed to couple, both systems become large-gap QAH insulators with fully spin-polarized half-metallic edge states and higher Fermi velocity of 4.9 × 105 m s-1. These results pave a new way for designing topological field transistors in group-IV honeycomb lattices.

Robust half-metallicity in transition metal tribromide nanowires.

One-dimensional (1D) nanowires (NWs) with robust half-metallicity are a rising star in spintronics. Herein, we theoretically investigate the magnetic and electronic properties of 3d transition-metal tribromide NWs, i.e. TMBr3 (TM = Sc, Ti, V, Cr, Co, and Cu). These systems represent repeated TMBr3 motifs with octahedral configuration, and are expected to be synthesized in a nanotube using an established method. Among these NWs, both VBr3 and CuBr3 NWs exhibit a ferromagnetic (FM) ground state, accompanied by sizable magnetocrystalline anisotropic energy, which is dominated by the superexchange coupling between the TM atoms. Strikingly, a half-metallic nature with a magnetic moment of 4.0µB per unit cell is predicted for the VBr3 NW. By combining with a tight-binding model, we demonstrate that the origin behind the half-metallicity is the half-filled e2 orbitals of the V atoms. The Curie temperature is evaluated to be up to 80 K using Monte Carlo simulations, which is comparable to the temperature of liquid nitrogen. We also find that the half-metallic behavior shows a favorable tolerance to the longitudinal elongation of the wire (∼10%). Additionally, a transition from FM semiconductor to half-metal can be realized in the CuBr3 NW through carrier doping. The coexistence of intrinsic high-temperature FM ordering and half-metallicity endows 1D TMBr3 NWs with great promise for spintronic and photoelectron device applications.

Bismuth oxide film: a promising room-temperature quantum spin Hall insulator.

Two-dimensional (2D) bismuth films have attracted extensive attention due to their nontrivial band topology and tunable electronic properties for achieving dissipationless transport devices. The experimental observation of quantum transport properties, however, are rather challenging, limiting their potential application in nanodevices. Here, we predict, based on first-principles calculations, an alternative 2D bismuth oxide, BiO, as an excellent topological insulator (TI), whose intrinsic bulk gap reaches up to 0.28 eV. Its nontrivial topology is confirmed by topological invariant Z 2 and time-reversal symmetry protected helical edge states. The appearance of topological phase is robust against mechanical strain and different levels of oxygen coverage in BiO. Since the BiO is naturally stable against surface oxidization and degradation, these results enrich the topological materials and present an alternative way to design topotronics devices at room temperature.

Discovery of a novel spin-polarized nodal ring in a two-dimensional HK lattice.

Nodal-ring materials with a spin-polarized feature have attracted intensive interest recently due to their exotic properties and potential applications in spintronics. However, such a type of two-dimensional (2D) lattice is rather rare and difficult to realize experimentally. Here, we identify the first 2D Honeycomb-Kagome (HK) lattice, Mn-Cyanogen, as a new single-spin nodal-ring material by using first-principles calculations. Mn-Cyanogen shows gapless and semiconducting properties in spin-up and spin-down orientations, respectively, indicating a spin-gapless semiconductor nature. Remarkably, a spin-polarized nodal ring induced by px,y/pz band inversion is captured from the 3D band structure, which is irrelevant to spin-orbit coupling. The origin of the single-spin nodal-ring can be further clarified by the effective tight-binding (TB) model. These results open a new avenue to achieving spin-polarized nodal-ring materials with promising applications in spintronic devices.

Effect of Amidogen Functionalization on Quantum Spin Hall Effect in Bi/Sb(111) Films.

Knowledge about chemical functionalization is of fundamental importance to design novel two-dimensional topological insulators. Despite theoretical predictions of quantum spin Hall effect (QSH) insulator via chemical functionalization, it is quite challenging to obtain a high-quality sample, in which the toxicity is also an important factor that cannot be ignored. Herein, using first-principles calculations, we predict an intrinsic QSH effect in amidogen-functionalized Bi/Sb(111) films (SbNH2 and BiNH2), characterized by nontrivial Z2 invariant and helical edge states. The bulk gaps derived from px,y orbitals reaches up to 0.39 and 0.83 eV for SbNH2 and BiNH2 films, respectively. The topological properties are robust against strain engineering, electric field, and rotation angle of amidogen, accompanied with sizable bulk gaps. Besides, the topological phases are preserved with different arrangements of amidogen. The H-terminated SiC(111) is verified as a good candidate substrate for supporting the films without destroying their QSH effect. These results have substantial implications for theoretical and experimental studies of functionalized Bi/Sb films, which also provide a promising platform for realizing practical application in dissipationless transport devices at room temperature.

Unconventional band inversion and intrinsic quantum spin Hall effect in functionalized group-V binary films.

Adequately understanding band inversion mechanism, one of the significant representations of topological phase, has substantial implications for design and regulation of topological insulators (TIs). Here, by identifying an unconventional band inversion, we propose an intrinsic quantum spin Hall (QSH) effect in iodinated group-V binary (ABI2) monolayers with a bulk gap as large as 0.409 eV, guaranteeing its viable application at room temperature. The nontrivial topological characters, which can be established by explicit demonstration of Z2 invariant and gapless helical edge states, are derived from the band inversion of antibonding states of p x,y orbitals at the K point. Furthermore, the topological properties are tunable under strain engineering and external electric field, which supplies a route to manipulate the spin/charge conductance of edge states. These findings not only provide a new platform to better understand the underlying origin of QSH effect in functionalized group-V films, but also are highly desirable to design large-gap QSH insulators for practical applications in spintronics.