Run-wu Zhang

Ethynyl-functionalized stanene film: a promising candidate as large-gap quantum spin Hall insulator

Quantum spin Hall (QSH) effect is promising for achieving dissipationless transport devices which can be achieved only at extremely low temperature presently. The research for new large-gap QSH insulators is critical for their realistic applications at room temperature. Based on first-principles calculations, we propose a QSH insulator with a sizable bulk gap as large as ~0.22 eV in stanene film functionalized with the organic molecule ethynyl (SnC2H), whose topological electronic properties are highly tunable by the external strain. This large-gap is mainly due to the result of the strong spin–orbit coupling related to the pxy orbitals at the Γ point of the honeycomb lattice, significantly different from that consisting of the pz orbital as in free-standing group IV ones. The topological characteristic of SnC2H film is confirmed by the Z2 topological order and an explicit demonstration of the topological helical Dirac type edge states. The SnC2H film on BN substrate is observed to support a nontrivial large-gap QSH, which harbors a Dirac cone lying within the band gap. Owing to their high structural stability, this two-dimensional large-gap QSH insulator is promising platforms for topological phenomena and new quantum devices operating at room temperature in spintronics.

Unexpected Giant-Gap Quantum Spin Hall Insulator in Chemically Decorated Plumbene Monolayer.

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.

New family of room temperature quantum spin Hall insulators in two-dimensional germanene films

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.

Room Temperature Quantum Spin Hall Insulator in Ethynyl-Derivative Functionalized Stanene Films

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.

Silicon-based chalcogenide: Unexpected quantum spin Hall insulator with sizable band gap

Quantum spin Hall (QSH) effect is promising for achieving dissipationless transport devices due to the robust gapless states inside insulating bulk gap. Here, by using first-principles calculations, we discover group-IV chalcogenide Si2Te2 film to be a 2D QSH insulator with a fundamental band gap of 0.29 eV, which is 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 locating in the bulk gap. Notably, the characteristic properties of edge states, such as the Fermi velocity and edge shape, can be tuned by edge modifications. Additionally, the h-BN semiconductor is an ideal substrate for experimental realization of 2D Si2Te2 film, without destroying its nontrivial topology. Our works open a new route for designing topological spintronics devices based on 2D silicon-based films.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.

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.

Functionalized Thallium Antimony Films as Excellent Candidates for Large-Gap Quantum Spin Hall Insulator.

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.

First-principles study on ferromagnetism in W-doped graphene

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 (<3.0 A), the systems converge to paramagnetic states due to their direct interactions between W defects. These findings are helpful for better understanding the origin of FM order in 4d or 5d transitions metal doped graphene.

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.

First-principles prediction of a giant-gap quantum spin Hall insulator in Pb thin film

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.