Miao-juan Ren

Controllable band structure and topological phase transition in two-dimensional hydrogenated arsenene

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.

Tunable Quantum Spin Hall Effect via Strain in two-Dimensional Arsenene Monolayer

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.

First-principles study of small Pd–Au alloy clusters on graphene

We performed extensive density functional theory (DFT) calculations of palladium (Pd) and gold (Au) alloy clusters adsorbed on a graphene monolayer in order to clarify the geometries and charge transfer of Pd–Au bimetal alloy clusters on graphene. It is found that the Pd–Au cluster prefers to bind with graphene through Pd atoms, with strong p-d hybridization between graphene and Pd atoms. Although the gold atom has an unpaired electron, the magnetic moments are mainly contributed by palladium. Compared with Pd–Au bimetal, the bonds between Au atoms are stronger; therefore, the gold atoms form a gold cap covering the Pd cluster. Furthermore, Bader charge analysis demonstrates that Pd in alloy clusters tends to lose electrons, and the number of charge transfers increases with the introduction of the graphene monolayer. Gold atoms and graphene synergistically improve electron loss on the Pd atom, thus weakening the adsorption of anions, which is expected to prevent poisoning of Pd nanocatalysts and enhance the catalytic reactivity of alloy clusters. However, the Au–Au coupling could weaken their ability to gain electrons from Pd significantly. So, an important task for experimental research is to find a way to disperse gold atoms as far apart as possible to improve the catalytic properties of the Pd–Au alloy cluster.

First-principles study of AlN nanosheets with chlorination

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.

Stanene cyanide: a novel candidate of Quantum Spin Hall insulator at high temperature.

The search for quantum spin Hall (QSH) insulators with high stability, large and tunable gap and topological robustness, is critical for their realistic application at high temperature. Using first-principle calculations, we predict the cyanogen saturated stanene SnCN as novel topological insulators material, with a bulk gap as large as 203 meV, which can be engineered by applying biaxial strain and electric field. The band topology is identified by Z2 topological invariant together with helical edge states, and the mechanism is s-pxy band inversion at G point induced by spin-orbit coupling (SOC). Remarkably, these systems have robust topology against chemical impurities, based on the calculations on halogen and cyano group co-decorated stanene SnXxX′1−x (X,X′  =  F, Cl, Br, I and CN), which makes it an appropriate and flexible candidate material for spintronic devices.

Giant gap quantum spin Hall effect and valley-polarized quantum anomalous Hall effect in cyanided bismuth bilayers

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.

Prediction of half-metallic ferromagnetism in C-doped CdS nanowire

The geometric, energetic, and magnetic properties of nonmagnetic carbon (C)-doped cadmium sulfur (CdS) nanowires (NWs) are investigated based on first-principles calculations. We find that the two C dopants are most stable when they are close to each other in the surface sites, exhibiting half-metallic (HM) behavior with a net magnetic moment of 2.0 μB C−1. The magnetic interaction between the nearest and next-nearest C dopants results in a strong ferromagnetic (FM) coupling. However, the ground state of the system tends to be paramagnetic when the distance between the C dopants is larger than 6.2 A. The HMFM order in C-doped CdS NWs, which is sensitive to the confinement of electrons in the radial direction and the curvature of the NWs surface, can be attributed to hole-mediated double exchange through the strong p–p interaction between carbon dopants. These predicted results endow CdS NWs potential applications in spintronics.

First-principles prediction and Monte Carlo study of half-metallic ferromagnetism of the C-doped honeycomb CdS monolayer

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.

First-principles prediction of inversion-asymmetric topological insulator in hexagonal BiPbH monolayer

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.

Quantum spin Hall state in cyanided dumbbell stanene

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.