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Hydrogen, a simple and magic element, has attracted increasing attention for its effective incorporation within solids and powerful manipulation of electronic states. Here, we show that hydrogenation tackles common problems in two-dimensional borophene, e.g., stability and applicability. As a prominent example, a ladder-like boron hydride sheet, named as 2D ladder polyborane, achieves the desired outcome, enjoying the cleanest scenario with an anisotropic and tilted Dirac cone, that can be fully depicted by a minimal two-band tight-binding model. Introducing external fields, such as an electric field or a circularly polarized light field, can effectively induce distinctive massive Dirac fermions, whereupon four types of multi-field-driven topological domain walls hosting tunable chirality and valley indexes are further established. Moreover, the 2D ladder polyborane is thermodynamically stable at room temperature and supports highly switchable Dirac fermions, providing an ideal platform for realizing and exploring the various multi-field-tunable electronic states.

Hydrogen, a simple and magic element, has attracted increasing attention for its effective incorporation within solids and powerful manipulation of electronic states. Here, we show that hydrogenation tackles common problems in two-dimensional borophene, e.g., stability and applicability. As a prominent example, a ladder-like boron hydride sheet, named as 2D ladder polyborane, achieves the desired outcome, enjoying the cleanest scenario with an anisotropic and tilted Dirac cone, that can be fully depicted by a minimal two-band tight-binding model. Introducing external fields, such as an electric field or a circularly-polarized light field can effectively induce distinctive massive Dirac fermions, whereupon four types of multi-field-driven topological domain walls hosting tunable chirality and valley indexes are further established. Moreover, the 2D ladder polyborane is thermodynamically stable at room temperature and supports highly switchable Dirac fermions, providing an ideal platform for realizing and exploring the various multi-field-tunable electronic states., Comment: 25 pages, 5 figures

Nodal monoloop, enjoying the cleanest scenario with a single loop, is recognized as the basic building block of intricate linked loops including chains, nets, and knots. Here, we explore the interplay of magnetic ordering and band topology in one system by introducing a brand-new quantum state, referred to as Weyl monoloop semi-half-metal, which is characterized by a single loop at the Fermi level stemming from the same spin channel. Such a nodal line Fermion, yielding 100% spin polarization, is protected by mirror ( M z ) symmetry. As a prominent example, a realistic rutile-type metal fluorides LiV2F6 achieves the hitherto unmaterialized state, featuring fully spin-polarized ultraflat surface states. More interestingly, LiV2F6 has a "soft" ferromagnetic property, which is one of the desired systems to control the anomalous Hall effect by rotating the magnetization direction. Our findings offer a promising candidate for exploring the topology and magnetism with intriguing effects.

Recently discovered high-quality nodal chain spin-gapless semimetals $M$F$_3$ ($M$ = Pd, Mn) feature an ultra-clean nodal chain in the spin up channel residing right at the Fermi level and displaying a large spin gap leading to a 100\% spin-polarization of transport properties. Here, we investigate both intrinsic and extrinsic contributions to anomalous and spin transport in this class of materials. The dominant intrinsic origin is found to originate entirely from the gapped nodal chains without the entanglement of any other trivial bands. The side-jump mechanism is predicted to be negligibly small, but intrinsic skew-scattering enhances the intrinsic Hall and Nernst signals significantly, leading to large values of respective conductivities. Our findings open a new material platform for exploring strong anomalous and spin transport properties in magnetic topological semimetals., 7 pages, 4 figures

The past decade has witnessed a surge of interest in exploring emergent particles in condensed matter systems. Novel particles, emerged as excitations around exotic band degeneracy points, continue to be reported in real materials and artificially engineered systems, but so far, we do not have a complete picture on all possible types of particles that can be achieved. Here, via systematic symmetry analysis and modeling, we accomplish a complete list of all possible particles in time reversal-invariant systems. This includes both spinful particles such as electron quasiparticles in solids, and spinless particles such as phonons or even excitations in electric-circuit and mechanical networks. We establish detailed correspondence between the particle, the symmetry condition, the effective model, and the topological character. This obtained encyclopedia concludes the search for novel emergent particles and provides concrete guidance to achieve them in physical systems., Comment: 5 pages for main text and 1228 pages for SM; comments are welcome, please contact Z. M. Yu, Y. Yao, or S. A. Yang

Topological semimetals in ferromagnetic materials have attracted enormous attention due to the potential applications in spintronics. Using the first-principles density functional theory together with an effective lattice model, here we present a new family of topological semimetals with a fully spin-polarized nodal loop in alkaline-metal monochalcogenide $MX$ ($M$ = Li, Na, K, Rb, Cs; $X$ = S, Se, Te) monolayers. The half-metallic ferromagnetism can be established in $MX$ monolayers, in which one nodal loop formed by two crossing bands with the same spin components is found at the Fermi energy. This nodal loop half-metal survives even when considering the spin-orbit coupling owing to the symmetry protection provided by the $\mathcal{M}_{z}$ mirror plane. The quantum anomalous Hall state and Weyl-like semimetal in this system can be also achieved by rotating the spin from the out-of-plane to the in-plane direction. The $MX$ monolayers hosting rich topological phases thus offer an excellent materials platform for realizing the advanced spintronics concepts.

Manipulating the spin degrees of freedom of electrons affords an excellent platform for exploring novel quantum states in condensed-matter physics and material science. Based on first-principles calculations and analysis of crystal symmetries, we propose a fully spin-polarized composite semimetal state, which is combined with the one-dimensional nodal lines and two-dimensional nodal surfaces, in the half-metal material CaFeO$_3$. In the nodal line-surface states, the Baguenaudier-like nodal lines feature six rings linked together, which are protected by the three independent symmetry operations:$\mathcal{PT}$, $\mathcal{M}_{y}$, and $\mathcal{\widetilde{M}}_{z}$. Near the Fermi level, the spin-polarized nodal surface states are guaranteed by the joint operation $\mathcal{T}\mathcal{S}_{2i}$ in the $k_{i(i=x,y,z)}=\pi$ plane. Furthermore, high-quality CaFeO$_3$ harbors ultra-clean energy dispersion, which is rather robust against strong triaxial compressional strain and correlation effect. The realization of the Weyl nodal line-surface half-metal presents great potential for spintronics applications with high speed and low power consumption., Comment: 5 figures + 1 table

The bottleneck of current studies on topological insulators is to identify better materials that can be fabricated into devices more feasibly. To search for novel topological materials, we developed a high-throughput framework that can be utilized to screen for candidates with known crystal structures and further showcase topological properties based on automated construction of Wannier functions. We have applied our methods to ternary compounds of Bi, Sb, and nitrides as a representative sample. The topological properties are characterized by the surface states, verified by auxiliary evaluation of the Z2 topological invariant. We successfully identified seven topological insulators. Our work paves the way to design novel topological materials.

Van der Waals heterostructures of transition metal dichalcogenides with interlayer coupling offer an exotic platform to realize fascinating phenomena. Due to the type II band alignment of these heterostructures, electrons and holes are separated into different layers. The localized electrons induced doping in one layer, in principle, would lift the Fermi level to cross the spin-polarized upper conduction band and lead to strong manipulation of valley magnetic response. Here, we report the significantly enhanced valley Zeeman splitting and magnetic tuning of polarization for the direct optical transition of MoS2 in MoS2/WS2 heterostructures. Such strong enhancement of valley magnetic response in MoS2 stems from the change of the spin-valley degeneracy from 2 to 4 and strong many-body Coulomb interactions induced by ultrafast charge transfer. Moreover, the magnetic splitting can be tuned monotonically by laser power, providing an effective all-optical route towards engineering and manipulating of valleytronic devices and quantum-computation. Van der Waals heterostructures may offer a suitable platform for all-optical manipulation of valleytronic devices. Here, the authors observe a strong enhancement of the valley magnetic response in MoS2, and magnetic tuning of the polarization of MoS2 direct optical transition

Spin-gapless semimetals (SGSMs), which generate 100\% spin polarization, are viewed as promising semi-half-metals in spintronics with high speed and low consumption. We propose and characterize a new $\mathbb{Z_{\mathrm{2}}}$ class of topological nodal line (TNL) in SGSMs. The proposed TNLSGSMs are protected by space-time inversion symmetry or glide mirror symmetry with two-dimensional (2D) fully spin-polarized nearly flat surface states. Based on first-principles calculations and effective model analysis, a series of high-quality materials with $\textit{R}\overline{3}\textit{c}$ and $\textit{R}{3}\textit{c}$ space groups are predicted to realize such TNLSGSMs (chainlike). The 2D fully spin-polarized nearly flat surface states may provide a route to achieving equal spin pairing topological superconductivity as well as topological catalysts., 6 pages, 4 figures

Two-dimensional (2D) large band-gap topological insulators (TIs) with highly stable structures are imperative for achieving dissipationless transport devices. However, to date, only very few materials have been experimentally observed to host the quantum spin Hall (QSH) effect at low temperature, thus obstructing their potential application in practice. Using first-principles calculations, herein, we predicted a new 2D TI in the porous allotrope of a bismuth monolayer, i.e. bismuthylene, its geometrical stability was confirmed via phonon spectrum and molecular dynamics simulations. Analysis of the electronic structures reveals that bismuthylene is a native QSH state with a band gap as large as 0.28 eV at the Γ point, which is smaller than that (0.50 eV) of the buckled Bi (111) and suitable for room temperature applications. Notably, it has a much lower energy than flattened Bi and a higher energy than buckled Bi (111)…” [corrected] and flattened Bi films; thus, bismuthylene is feasible for experimental realization. Interestingly, the topological properties can be retained under strains within the range of -6%-3% and electrical fields up to 0.8 eV A-1. A heterostructure was constructed by sandwiching bismuthylene between BN sheets, and the non-trivial topology of bismuthylene was retained with a sizable band gap. These findings provide a platform to design a large-gap QSH insulator based on the 2D bismuthylene films, which show potential applications in spintronic devices.

Two-dimensional (2D) materials have attracted great attention and spurred rapid development in both fundamental research and device applications. The search for exotic physical properties, such as magnetic and topological order, in 2D materials could enable the realization of novel quantum devices and is therefore at the forefront of materials science. Here, we report the discovery of two-fold degenerate Weyl nodal lines in a 2D ferromagnetic material, a single-layer gadolinium-silver compound, based on combined angle-resolved photoemission spectroscopy measurements and theoretical calculations. These Weyl nodal lines are symmetry protected and thus robust against external perturbations. The coexistence of magnetic and topological order in a 2D material is likely to inform ongoing efforts to devise and realize novel nanospintronic devices., 4 figures

Two-dimensional (2D) topological insulators (TIs) have attracted tremendous research interest from both the theoretical and the experimental fields in recent years. However, it is much less investigated in realizing node line (NL) semimetals in 2D materials. Combining first-principles calculations and symmetry analysis, we find that NL phases emerge in $p\text{\ensuremath{-}}{\mathrm{CS}}_{2}$ and $p\text{\ensuremath{-}}{\mathrm{SiS}}_{2}$, as well as other pentagonal ${\mathrm{IVX}}_{2}$ films, i.e., $p\text{\ensuremath{-}}{\mathrm{IVX}}_{2}$ (IV= C, Si, Ge, Sn, Pb; X=S, Se, Te) in the absence of spin-orbit coupling (SOC). The NLs in $p\text{\ensuremath{-}}{\mathrm{IVX}}_{2}$ consist of symbolic Fermi loops centered around the $\mathrm{\ensuremath{\Gamma}}$ point and are protected by mirror reflection symmetry. As the atomic number is downward shifted, the NL semimetals are driven into 2D TIs with the large bulk gap up to 0.715 eV induced by the remarkable SOC effect. The nontrivial bulk gap can be tunable under external biaxial strain and uniaxial strain. Moreover, we also propose a quantum well by sandwiching a $p\text{\ensuremath{-}}{\mathrm{PbTe}}_{2}$ crystal between two NaI sheets in which $p\text{\ensuremath{-}}{\mathrm{PbTe}}_{2}$ still keeps its nontrivial topology with a sizable band gap ($\ensuremath{\sim}0.5$ eV). These findings provide a new 2D material platform for exploring fascinating physics in both NL semimetals and TIs.

Searching for two-dimensional (2D) group IV films with high structural stability and large-gaps is crucial for the realization of a dissipationless transport edge state using the quantum spin Hall effect (QSHE). Based on first-principles calculations, we predict that 2D germanene decorated with ethynyl-derivatives (GeC2X; X = H, F, Cl, Br, I) can be a topological insulator (TI) with a large band-gap for room-temperature applications. Both GeC2I and GeC2Br films are intrinsic TIs with a gap reaching up to 180 meV over a wide range, while GeC2H, GeC2F, and GeC2Cl transform from trivial to nontrivial phases under tensile strain. This topological characteristic can be confirmed by s–pxy band inversion, topological invariant Z2, and time-reversal symmetry protected helical edge states. Notably, the characteristic properties of edge states, such as the Fermi velocity and edge shape, can be tuned by edge modifications. Furthermore, we demonstrate that the h-BN sheet is an ideal substrate for the experimental realization of GeC2X, maintaining their nontrivial topology. Considering their higher thermo-stability, these GeC2X films may be good QSHE platforms for topological electronic device design and fabrication in spintronics.

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.

The quantum spin Hall (QSH) effect is promising for achieving dissipationless transport devices due to the robust gapless states inside the insulating bulk gap. However, QSH insulators currently suffer from requiring extremely high vacuums or low temperatures. Here, using first-principles calculations, we predict cyanogen-decorated plumbene (PbCN) to be a new QSH phase, with a large gap of 0.92 eV, that is robust and tunable under external strain. The band topology mainly stems from s–pxy band inversion related to the lattice symmetry, while the strong spin–orbit coupling (SOC) of the Pb atoms only opens a large gap. When halogen atoms are incorporated into PbCN, the resulting inversion-asymmetric PbFx(CN)1−x can host the QSH effect, accompanied by the presence of a sizable Rashba spin splitting at the top of the valence band. Furthermore, the Te(111)-terminated BaTe surface is proposed to be an ideal substrate for experimental realization of these monolayers, without destroying their nontrivial topology. These findings provide an ideal platform to enrich topological quantum phenomena and expand the potential applications in high-temperature spintronics.

The structural and electronic properties of silicene/silicane and silicene/germanene heterobilayers (HBLs) are investigated by using first-principles methods. The results show that the silicene interacts overall with silicane (germanene) with a binding energy of −50∼−70 meV per Si (Ge) atom, suggesting a weakly van der Waals interaction between silicene and substrate. A relative large bandgap with a linear band dispersion of HBLs is opened due to the sublattice symmetry broken by the intrinsic interface dipole between silicene and substrate. Remarkably, the band gap of all these HBLs can also be continually tuned modulated by adjusting the interlayer spacing and strain, independent on the stacking arrangements. Silicene is thus expected to be useful for building high-performance FET channel, which would extend its applicability to possible future nanoelectronics.

Based on first-principles calculations and symmetry analysis, we report that the three-dimensional (3D) nodal line (NL) semimetal phases can be realized in the lead dioxide family (\emph{$\alpha$}-PbO$_2$ and \emph{$\beta$}-PbO$_2$) and its derivatives. The \emph{$\beta$}-PbO$_2$ features two orthogonal nodal rings around the Fermi level, protected by the mirror reflection symmetry. The effective model is developed and the related parameters are given by fitting with the HSE06 band structures. The NLs mainly come from the $p$ orbitals of the light element O and are rather robust against such tiny spin-orbit coupling. The NL phase of the \emph{$\alpha$}-PbO$_2$ can be effectively tailored by strain, making a topological phase transition between a semiconductor phase and a NL phase. In addition, the exploration of \emph{$\beta$}-PbO$_2$ derivatives (i.e. \emph{$\beta$}-PbS$_2$ and \emph{$\beta$}-PbSe$_2$) and confirmation of their topological semimetallicity greatly enrich the NL semimetal family. These findings pave a route for designing topological NL semimetals and spintronic devices based on realistic PbO$_2$ family., Comment: 7 pages, 7 figures

It is challenging to epitaxially grow germanene on conventional semiconductor substrates. Based on first-principles calculations, we investigate the structural and electronic properties of germanene/germanane heterostructures (HTSs). The results indicate that the Dirac cone with nearly linear band dispersion of germanene is maintained in the band gap of the substrate. Remarkably, the band gaps opened in these HTSs can be effectively modulated by the external electric field and strain, and they also feature very low effective masses and high carrier mobilities. These results provide a route to design high-performance FETs operating at room temperature in nanodevices.

Very recently, graphene/SnO2 heterostructures (G/SnO2 HTSs) were successfully synthesized experimentally. Motivated by this work, the adhesion and electronic properties of G/SnO2 HTSs have been studied by using first-principles calculations. It is found that the graphene interacts overall with the SnO2 monolayer with a binding energy of −67–−70 meV per carbon atom, suggesting a weak van der Waals interaction between graphene and the SnO2 substrate. Although the global band gap is zero, a sizable band gap of 10.2–12.6 meV at the Dirac point is obtained in all G/SnO2 HTSs, mainly determined by the distortion of isolated graphene peeled from the SnO2 monolayer, independent of the SnO2 substrate. When the bilayer graphene is deposited on the SnO2 substrate, however, a global gap of 100 meV is formed at the Fermi level, which is large enough for the gap opening at room temperature. Interestingly, the characteristics of the Dirac cone with a nearly linear band dispersion relation of graphene can be preserved, accompanied by a small electron effective mass, and thus higher carrier mobility is expected. These finds provide a better understanding of the interfacial properties of G/SnO2 HTSs and will help to design high-performance FETs in nanoelectronics.

Opening up a band gap without lowering high carrier mobility and finding a suitable substrate material are a challenge for designing silicon-based nanodevices. Using density functional theory calculations incorporating vdW corrections, we find that the semiconducting silicane monolayer is free of dangling bonds, providing an ideal substrate for silicene to sit on. The nearly linear band dispersion character of silicene with a sizable band gap (44–61 meV) opening is obtained in all heterobilayers (HBLs). We also find that the effective masses of electrons and holes near the Dirac point (ranging from 0.033 to 0.045m0) are very small in HBLs, and thus high carrier mobility (105cm2 V–1 s–1) of silicene is expected. These characteristics of HBLs can be flexibly modulated by applying bias voltage or strain, suitable for the high-performance FET channel operating at room temperature.

In this theoretical work, we perform first-principles calculation to study the geometric, electronic, and magnetic properties of the two-dimensional germanene and silicene hybrid sheet (GeSiHS) saturated with brominate atoms (Br). Although the pristine GeSiHS is semi-metallic, half-saturation on only a single side of Si atoms with Br results in the localized and unpaired electrons in unsaturated Ge atoms, showing long-range ferromagnetic (FM) properties with a high Curie temperature. Interestingly, half-brominated GeSiHS exhibits half-metallic character with 100% spin-polarized currents at the Fermi level. These findings provide the possibility of GeSiHS as promising building blocks for spintronic devices.

Searching for realistic materials able to realize room-temperature quantum spin Hall (QSH) effects is currently a growing field, especially when compatibility with the current group-IV electronics industry is required. Here we predict, through first-principles calculations, a new class of QSH phases in flattened germanene and stanene functionalized with X atoms (f-GeX2 and f-SnX2; X = H, F, Cl, Br, I) with a bulk gap as large as 0.56 eV, that can be tuned efficiently under mechanical strain. More interestingly, different from the normal band order in buckled germanane and stanane, the structural flatness leads to an inverted band order without spin–orbit coupling (SOC), whereas the SOC only opens the band gap. We also find that the characteristics of edge states, such as the Fermi velocity, are enhanced greatly by edge modification. When these films are deposited on a BN substrate, a nontrivial QSH state is preserved with a Dirac cone lying within the nontrivial band gap. These findings provide a promising platform for future realistic applications of the QSH effect at room temperature in two-dimensional group-IV films.

Group III-V films are of great importance for their potential application in spintronics and quantum computing. Search for two-dimensional III-V films with a nontrivial large-gap are quite crucial for the realization of dissipationless transport edge channels using quantum spin Hall (QSH) effects. Here we use first-principles calculations to predict a class of large-gap QSH insulators in functionalized TlSb monolayers (TlSbX2; (X = H, F, Cl, Br, I)), with sizable bulk gaps as large as 0.22~0.40 eV. The QSH state is identified by Z2 topological invariant together with helical edge states induced by spin-orbit coupling (SOC). Noticeably, the inverted band gap in the nontrivial states can be effectively tuned by the electric field and strain. Additionally, these films on BN substrate also maintain a nontrivial QSH state, which harbors a Dirac cone lying within the band gap. These findings may shed new light in future design and fabrication of QSH insulators based on two-dimensional honeycomb lattices in spintronics., 20 pages, 6 figures

Quantum spin Hall (QSH) insulators feature edge states that topologically protected from backscattering. However, the major obstacles to application for QSH effect are the lack of suitable QSH insulators with a large bulk gap. Based on first-principles calculations, we predict a class of large-gap QSH insulators in ethynyl-derivative functionalized stanene (SnC2X; X = H, F, Cl, Br, I), allowing for viable applications at room temperature. Noticeably, the SnC2Cl, SnC2Br and SnC2I are QSH insulators with a bulk gap of ~0.2 eV, while the SnC2H and SnC2F can be transformed into QSH insulator under the tensile strains. A single pair of topologically protected helical edge states is established for the edge of these systems with the Dirac point locating at the bulk gap and their QSH states are confirmed with topological invariant Z2 = 1. The films on BN substrate also maintain a nontrivial large-gap QSH effect, which harbors a Dirac cone lying within the band gap. These findings may shed new light in future design and fabrication of large-gap QSH insulators based on two-dimensional honeycomb lattices in spintronics.

The search for a new quantum spin Hall (QSH) phase and effective manipulation of its edge states are very important for both fundamental sciences and practical applications. Here, we use first-principles calculations to study the strain-driven topological phase transition of two-dimensional (2D) arsenene monolayer. We find that the band gap of arsenene decreases with increasing strain and changes from indirect to direct, and then the s-p band inversion takes place at the Г point as the tensile strain is larger than 11.14%, which leads to a nontrivially topological state. A single pair of topologically protected helical edge states is established for the edge of arsenene, and their QSH states are confirmed with the nontrivial topological invariant Z 2 = 1. We also propose high-dielectric BN as an ideal substrate for the experimental synthesis of arsenene, maintaining its nontrivial topology. These findings provide a promising candidate platform for topological phenomena and new quantum devices operating at nanoelectronics.

Quantum spin Hall (QSH) effect of two-dimensional (2D) materials features edge states that are topologically protected from backscattering by time-reversal symmetry. However, the major obstacles to the application for QSH effect are the lack of suitable QSH insulators with a large bulk gap. Here, we predict a novel class of 2D QSH insulators in X-decorated plumbene monolayers (PbX; X = H, F, Cl, Br, I) with extraordinarily giant bulk gaps from 1.03 eV to a record value of 1.34 eV. The topological characteristic of PbX mainly originates from s-px,y band inversion related to the lattice symmetry, while the effect of spin-orbital coupling (SOC) is only to open up a giant gap. Their QSH states are identified by nontrivial topological invariant Z2 = 1, as well as a single pair of topologically protected helical edge states locating inside the bulk gap. Noticeably, the QSH gaps of PbX are tunable and robust via external strain. We also propose high-dielectric-constant BN as an ideal substrate for the experimental realization of PbX, maintaining its nontrivial topology. These novel QSH insulators with giant gaps are a promising platform to enrich topological phenomena and expand potential applications at high temperature.

Correction for ‘First-principles prediction on bismuthylene monolayer as a promising quantum spin Hall insulator’ by Run-Wu Zhang, et al., Nanoscale, 2017, 9, 8207–8212.

Searching for two-dimensional (2D) silicon-based topological materials is imperative for the development of various innovative devices. Here, by using first-principles calculations, we discover the silicon-based chalcogenide Si2Te2 film to be a 2D quantum spin Hall (QSH) insulator with a fundamental band gap of 0.34 eV, which can be tunable under external strain. This nontrivial topological phase stems from band inversion between the Si-px,y and Te-px,y orbitals, demonstrated by a single pair of topologically protected helical edge states with Dirac point located in the bulk gap. Notably, the characteristic properties of edge states, such as the Fermi velocity and edge shape, can be engineered by edge modifications. Additionally, the BN sheet is an ideal substrate for the experimental realization of Si2Te2 films, without destroying its nontrivial topology. Our works open a meaningful route for designing topological spintronics devices based on 2D silicon-based films.

Efficient control and manipulation of spin degrees of freedom without a magnetic field is one of the challenges in developing spintronic devices. Here, we propose a new class of bipolar magnetic semiconductor (BMS) from semihydrogenated BAs nanosheets operated at a Curie temperature of 307 K. Both electron and hole self-doping on the structure with semihydrogenated As atoms induce the transition from ferromagnetic semiconductor to half-metal. The BMS nature is robust under the effect of strain or even a strong electric field. These findings highlight a promising new way toward electrical manipulation of the carrier's spin orientation in two-dimensional materials.

We perform first-principles calculations to study the geometric, electronic, and magnetic properties in SnO2 nanosheet (NS) with intrinsic oxygen (VO) or tin (VSn) defects. VO defect preserves the semiconducting behavior of pure SnO2NS, while VSn-induced magnetic state is relatively complex. The induced states mainly come from the unpaired electrons on oxygen atoms surrounding the tin vacancy. Depending on different initial spin distributions, we find three spin configurations with half-metallic state (S = 1), low spin semi-conductive state (S = 0) and high spin metallic state (S = 2). More importantly, there exists the long-range FM order between two 100% spin-polarized states, attributed to the hole-mediated double exchange through the strong p–d interaction between neighboring oxygen atoms. Our predicted diverse and intriguing magnetic properties induced by VSn defects provided an insight into the origin of ferromagnetism for designing new spintronics devices based on SnO2NS.

Based on first-principles calculations, we study the effects of the chlorine atoms on electronic and magnetic properties of AlN nanosheets (NS). We find that both the bare and fully-chlorinated AlNNRs demonstrate semiconducting behavior, while the half-chlorination on surface Al sites leads to the semiconductor-ferromagnetism transition. More interestingly, the chlorination on surface Al sites in monolayer and bilayer AlNNSs demonstrates the half-metallic ferromagnetic (FM) behavior with 100% spin-polarized currents at the Fermi level, suitable for applications in spintronics at the nanoscale.

Based on density-functional theory using the generalized gradient approximation plus Hubbard U scheme, we studied the structural, electronic, and magnetic properties of graphene doped with W atoms. Our results show that W introduces a spin polarized magnetic state with a local moment of 2.00 μB, which can be well understood using a hybridization model. When two W defects are introduced into graphene, the ferromagnetic (FM), antiferromagnetic, and paramagnetic states are obtained, depending on the crystal directions and relative positions between two W defects. Further analysis indicates that a Ruderman–Kittel–Kasuya–Yosida (RKKY) like behavior plays an important role in the magnetic order when the distance between W atoms is relatively large. However, when it is rather small (