Search

Nodal-line half-metals (NLHMs) are highly desirable for future spintronic devices due to their exotic quantum properties. However, the experimental realization in spin-polarized materials is nontrivial to date. Herein we perform first-principles calculations to demonstrate a 2D honeycomb, AgN, as a promising candidate of NLHMs, which is thermodynamically and dynamically stable. Band structure analysis reveals that two concentric NLs coexist centered at a Γ point near EF, accompanied by the electron and hole pockets that touch each other linearly with single-spin components. Inclusion of SOC can enrich the electronic structures of AgN, sensitive to the protection of mirror reflection symmetry: the NLHM survives if the spin is perpendicular to the Mz mirror plane, while it tunes into Wyle nodal-points by rotating spins from the out-of-plane to the in-plane direction. The characteristics of HM and NL can be well maintained on semiconducting h-BN and is immune to mechanical strains. These tunable magnetic properties render 2D AgN suitable for exotic quantum transports in nodal fermions as well as related spintronic devices.

A quantum spin Hall (QSH) insulator with high stability, large bulk band gap and tunable topological properties is crucial for both fundamental research and practical application due to the presence of dissipationless edge conducting channels. Recently, chemical functionalization has been proposed as an effective route to realize the QSH effect. Based on first-principles calculations, we predict that a two-dimensional TlP monolayer would convert into a topological insulator with the effect of bromination, accompanied by a large bulk band gap of 76.5 meV, which meets the requirement for room-temperature application. The topological nature is verified by the calculation of Z2 topological invariant and helical edge states. Meanwhile, an appreciable Rashba spin splitting of 77.2 meV can be observed. The bulk band gap can be effectively tuned with external strain and electric field, while the Rashba spin splitting shows a parabolic variation trend under an external electric field. We find that the topological property is available for the TlP film when the coverage rate is more than 0.75. BN and SiC are demonstrated as promising substrates to support the topological nature of TlPBr2 film. Our findings suggest that a TlPBr2 monolayer is an appropriate candidate for hosting the nontrivial topological state and controllable Rashba spin splitting, and shows great potential applications in spintronics.

Opening a sizable band gap in the zero-gap silicene is a key issue for its application in nanoelectronics. We design new 2D silicene and GaP heterobilayer (Si/GaP HBL) composed of silicene and monolayer (ML) GaP. Based on first-principles calculations, we find that the interaction energies are in the range of −295.5 to −297.5 meV per unit cell, indicating a weak interaction between silicene and gallium phosphide (GaP) monolayer. The band gap changes ranging from 0.06 to 0.44 eV in hybrid HBLs. An unexpected indirect–direct band gap crossover is also observed in HBLs, dependent on the stacking pattern. These provide a possible way to design effective FETs out of silicene on GaP monolayer.

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.

Band topology and Rashba spin splitting (RSS) are two extensively explored exotic properties in condensed matter physics. However, the coexistence has rarely been reported in simplest stoichiometric films so far. Here, by using first-principles calculations, we demonstrate that a series of inversion-asymmetric group-IV XYH2 monolayers (X, Y = Si, Ge, Sn, Pb) allow for the simultaneous presence of topological order and large RSS that derives from their peculiarly atomic structure. The topological bulk gaps and RSS energies of PbSnH2, PbGeH2, and PbSiH2 are tunable over a wide range of strains (−8 to 8%), even the maximum value can be enhanced to 0.68 eV and 0.24 eV under achievable strain, but another three configurations transform from trivial to nontrivial phases under tensile strain. Furthermore, we find that the Te(111)-terminated BaTe surface is an ideal substrate for the growth of these monolayers, without destroying their intrinsic band topology. Our findings provide a possible route to future applications of inversion-asymmetric topological insulators in spintronic devices.

Band topology and Rashba spin splitting (RSS) are two extensively explored yet exotic properties in condensed matter physics. Nonetheless, their coexistence has rarely been achieved in simple stoichiometric two-dimensional (2D) ultrathin films. Here we use first-principles calculations to predict a new inversion-asymmetric BiPbH monolayer, which allows for the simultaneous presence of nontrivially topological order and large RSS. Interestingly, the coexistence of the topological band gaps and RSS in this system are robust and stable over a wide range of strain (−6 to 6%), with the maximum bulk gap being enhanced to 0.40 eV and Rashba energy as large as 53.1 meV under achievable strains, respectively, which makes them viable for practical realization of the QSH state at room temperature. The nontrivial quantum state originated from pxy–pz band inversion is confirmed by Z2 index and helical edge states. Additionally, the h-BN semiconductor is an ideal substrate for experimental realization of BiPbH, without destroying its nontrivial topology. Our works open a new route for designing topological spintronics devices based on 2D inversion-asymmetric films.

Searching for two-dimensional (2D) quantum spin Hall (QSH) insulators with a large band gap, in which the Quantum spin Hall effect (QSHE) can be observed at high temperature, is an important goal for condensed matter physics researchers. Based on first principles calculations, we predict that the band gap of dumbbell stanene can be enhanced to 52 meV with cyano group decoration (DB-SnCN), which can be further tuned to be 322 meV by applying lattice strains. The band topology is identified by Z2 topological invariance together with helical edge states, and the mechanism can be understood with px–py band inversion at the Γ point induced by spin-orbit coupling (SOC). These findings provide bright and significant guidance for future fabrication and realistic applications of spintronics.

Based on first-principles hybrid functional calculations, we demonstrate the formation of two-dimensional (2D) topological insulators (TIs) of Pb/Sb honeycombs on Ge(111) semiconductor surface. We show that 1/3 Cl-covered Ge(111) surface offers an ideal template for metal deposition. When Pb and Sb atoms are deposited on Cl–Ge(111) surface, they spontaneously form a hexagonal lattice (Pb/Sb@Cl–Ge(111)). The Pb/Sb@Cl–Ge(111) exhibits a 2D TI state with large bulk gap of 0.27 eV for Pb@Cl–Ge(111) and 0.81 eV for Sb@Cl–Ge(111). The mechanism of 2D TI state is the substrate orbital-filtering effect that effectively removes the pz bands of Pb(Sb) away from the Fermi level, leaving behind only the px and py orbitals at the Fermi level. Our findings pave another way for future design of 2D topological insulators on conventional semiconductor surface, which promotes the application of 2D TIs in spintronics and quantum computing devices at room-temperature.

We use first-principles calculations to predict the occurrence of half-metallicity and anionogenic ferromagnetism at the heterointerface between two 2p insulators, taking the ${\mathrm{KO}}_{2}/{\mathrm{BaO}}_{2}$ (001) interface as an example. Whereas a sharp heterointerface is semiconducting, a heterointerface with a moderate concentration of swapped K and Ba atoms is half-metallic and ferromagnetic at ambient pressure due to the double exchange mechanism. The K-Ba swap renders the interfacial K-O and Ba-O atomic layers electron-doped and hole-doped, respectively. Our findings pave the way to realize metallicity and ferromagnetism at the interface between two $2p$ insulators, and such systems can constitute a new family of heterostructures with novel properties, expanding studies on heterointerfaces from $3d$ insulators to $2p$ insulators.

Using first-principles calculations, we studied the geometric and band structures of 20 possible configurations of buckled hydrogenated SnPb alloy (SnxPb8−xH8) films. The configurations are topological insulators (TIs) when x ≥ 1. When x increases from 1 to 7, the band gap increases from 0.087 eV to 0.98 eV. The topological characteristics are suggested by s–pxy band inversion and confirmed by helical edge states, which are time-reversal symmetry protected. According to spin–orbit coupling (SOC) analysis results, we draw the conclusion that the Pb atoms have greater SOC strength than the Sn atoms, so when the number ratio of Pb and Sn atoms is greater than 1/12 the SOC strength is large enough to trigger the band inversion between the s and pxy orbitals, causing the SnPb alloy film to turn into a topological insulator (TI) from a normal band insulator (NI). We give a simple rule for the topological criterion of hydrogenated SnPb alloy films by comparing the ratio of Pb and Sn atoms. This would provide a useful reference for the design of topological devices based on NI–TI hetero-junctions for experiments.

Correction for ‘Prediction of topological property in TlPBr2 monolayer with appreciable Rashba effect’ by Min Yuan et al., Phys. Chem. Chem. Phys., 2018, 20, 4308–4316.

Discovery of two-dimensional (2D) topological insulator such as group-V films initiates challenges in exploring exotic quantum states in low dimensions. Here, we perform first-principles calculations to study the geometric and electronic properties in 2D arsenene monolayer with hydrogenation (HAsH). We predict a new σ-type Dirac cone related to the px,y orbitals of As atoms in HAsH, dependent on in-plane tensile strain. Noticeably, the spin-orbit coupling (SOC) opens a quantum spin Hall (QSH) gap of 193 meV at the Dirac cone. A single pair of topologically protected helical edge states is established for the edges and its QSH phase is confirmed with topological invariant Z2 = 1. We also propose a 2D quantum well (QW) encapsulating HAsH with the h-BN sheet on each side, which harbors a nontrivial QSH state with the Dirac cone lying within the band gap of cladding BN substrate. These findings provide a promising innovative platform for QSH device design and fabrication operating at room temperature.

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.

By the full-potential linearized augmented plane wave (FLAPW) method, the electronic structures and magnetic properties of a C-doped honeycomb CdS monolayer (HC-CdS) have been investigated. We find that a C-doped HC-CdS shows a half-metallic character and a 100% spin polarization at the Fermi level with a total magnetic moment of 2.0 μB per unit cell. When two carbon atoms substitute for S atoms in HC-CdS, the interaction between two carbon atoms is antiferromagnetic, but it can turn into ferromagnetic coupling provided that an electron is doped into HC-CdS. The Curie temperature of 354 K is predicted through Monte Carlo simulation. The ferromagnetism of two C-doped HC-CdS monolayer can be explained by the electron-mediated p–p interaction and p–d exchange hybridization. These results provide a new perspective for the potential application of C-doped HC-CdS in spintronics.

Bismuth (Bi) has attracted a great deal of attention for its strongest spin–orbit coupling (SOC) strength among main group elements. Although quantum anomalous Hall (QAH) state is predicted in half-hydrogenated Bi honeycomb monolayers Bi2H, the experimental results are still missing. Halogen atoms (X = F, Cl and Br) were also frequently used as modifications, but Bi2X films show a frustrating metallic character that masks the QAH effects. Here, first-principle calculations are performed to predict the full-cyanided bismuthene (Bi2(CN)2) as 2D topological insulator supporting quantum spin Hall state with a record large gap up to 1.10 eV, and more importantly, half-cyanogen saturated bismuthene (Bi2(CN)) as a Chern insulator supporting a valley-polarized QAH state, with a Curie temperature to be 164 K, as well as a large gap reaching 0.348 eV which could be further tuned by bi-axial strain and SOC strength. Our findings provide an appropriate and flexible material family candidate for spintronic and valleytronic devices.