Quantum spin Hall insulators and topological Rashba-splitting edge states in two-dimensional CX3 (X = Sb, Bi)

Strain-free MoS2/ZrGe2N4 van der Waals Heterostructure: Tunable Electronic Properties with Type-II Band Alignment

Vertically stacked van der Waals heterostructures (vdW-HS) amplify the scope of 2D materials for emerging technological applications, such as nanodevices and solar cells. Here, we present a first-principles study on the formation energy and electronic properties of the heterobilayer (HBL) MoS2/ZrGe2N4, which forms a strain-free vdW-HS thanks to the identical lattice parameters of its constituents. This system has an indirect band gap with type-II band alignment, with the highest occupied and lowest unoccupied states localized on MoS2 and ZrGe2N4, respectively. Biaxial strain, which generally reduces the band gap regardless of compression or expansion, is applied to tune the electronic properties of the HBL. A small amount of tensile strain (>1%) leads to an indirect-to-direct transition, thereby shifting the band edges at the center of the Brillouin zone and leading to optical absorption in the visible region. These results suggest the potential application of HBL MoS2/ZrGe2N4 in optoelectronic devices.

Ideal two-dimensional quantum spin Hall insulators MgA2Te4 (A = Ga, In) with Rashba spin splitting and tunable properties

For decades, topological insulators have played a pivotal role in fundamental condensed-matter physics owing to their distinctive edge states and electronic properties. Here, based on in-depth first-principles calculations, we investigate the MgA2Te4 (A = Ga, In) structures belonging to the MA2Z4 2D material family. Among them, the topological insulator MgGaInTe4 exhibits band inversion and a sizeable bandgap of up to 60.8 meV which satisfies the requirement for room-temperature realization. Under the spin-orbit coupling effect, MgGaInTe4 with inversion asymmetry undergoes Rashba spin splitting. The Rashba-like and Dirac-type edge states emerge from different terminals along (010) for MgGaInTe4. The external vertical electric field is verified to modulate the inverted bandgap and topological state of MgGaInTe4 by converting a nontrivial state to a trivial state and MgIn2Te4 with the original trivial state to a nontrivial one. Accordingly, MgGaInTe4 and MgIn2Te4 have significant potential for application in topological quantum field-effect transistors. Our research identifies that the MgA2Te4 (A = Ga, In) structures have huge potential to be candidate 2D materials for spintronics and topological quantum devices.

Strained Monolayer MoTe2 as a Photon Absorber in the Telecom Range

To achieve the atomistic control of two-dimensional materials for emerging technological applications, such as valleytronics, spintronics, and single-photon emission, it is of paramount importance to gain an in-depth understanding of their structure-property relationships. In this work, we present a systematic analysis, carried out in the framework of density-functional theory, on the influence of uniaxial strain on the electronic and optical properties of monolayer MoTe2. By spanning a ±10% range of deformation along the armchair and zigzag direction of the two-dimensional sheet, we inspect how the fundamental gap, the dispersion of the bands, the frontier states, and the charge distribution are affected by strain. Under tensile strain, the system remains a semiconductor but a direct-to-indirect band gap transition occurs above 7%. Compressive strain, instead, is highly direction-selective. When it is applied along the armchair edge, the material remains a semiconductor, while along the zigzag direction a semiconductor-to-metal transition happens above 8%. The characteristics of the fundamental gap and wave function distribution are also largely dependent on the strain direction, as demonstrated by a thorough analysis of the band structure and of the charge density. Additional ab initio calculations based on many-body perturbation theory confirm the ability of strained MoTe2 to absorb radiation in the telecom range, thus suggesting the application of this material as a photon absorber upon suitable strain modulation.

Large-band-gap non-Dirac quantum spin Hall states and strong Rashba effect in functionalized thallene films

The quantum spin Hall state materials have recently attracted much attention owing to their potential applications in the design of spintronic devices. Based on density functional theory calculations and crystal field theory, we study electronic structures and topological properties of functionalized thallene films. Two different hydrogenation styles (Tl2H and Tl2H2) are considered, which can drastically vary the electronic and topological behaviors of the thallene. Due to the C3v symmetry of the two systems, the px and py orbitals at the Γ point have the non-Dirac band degeneracy. With spin-orbit coupling (SOC), topological nontrivial band gaps can be generated, giving rise to non-Dirac quantum spin Hall states in the two thallium hydride films. The nontrivial band gap for the monolayer Tl2H is very large (855 meV) due to the large on-site SOC of Tl px and py orbitals. The band gap in Tl2H2 is, however, small due to the band inversion between the Tl px/y and pz orbitals. It is worth noting that both the Tl2H and Tl2H2 monolayers exhibit strong Rashba spin splitting effects, especially for the monolayer Tl2H2 (αR = 2.52 eVÅ), rationalized well by the breaking of the structural inversion symmetry. The Rashba effect can be tuned sensitively by applying biaxial strain and external electric fields. Our findings provide an ideal platform for fabricating room-temperature spintronic and topological electronic devices.

Spin-valley-coupled quantum spin Hall insulator with topological Rashba-splitting edge states in Janus monolayer CSb1.5Bi1.5

Achieving combination of spin and valley polarized states with topological insulating phase is pregnant to promote the fantastic integration of topological physics, spintronics and valleytronics. In this work, a spin-valley-coupled quantum spin Hall insulator (svc-QSHI) is predicted in Janus monolayer CSb_1.5Bi_1.5with dynamic, mechanical and thermal stabilities. Calculated results show that the CSb_1.5Bi_1.5is a direct band gap semiconductor with and without spin-orbit coupling, and the conduction-band minimum and valence-band maximum are at valley point. The inequivalent valleys have opposite Berry curvature and spin moment, which can produce a spin-valley Hall effect. In the center of Brillouin zone, a Rashba-type spin splitting can be observed due to missing horizontal mirror symmetry. The topological characteristic of CSb_1.5Bi_1.5is confirmed by theZ₂invariant and topological protected conducting helical edge states. Moreover, the CSb_1.5Bi_1.5shows unique Rashba-splitting edge states. Both energy band gap and spin-splitting at the valley point are larger than the thermal energy of room temperature (25 meV) with generalized gradient approximation level, which is very important at room temperature for device applications. It is proved that the spin-valley-coupling and nontrivial quantum spin Hall state are robust again biaxial strain. Our work may provide a new platform to achieve integration of topological physics, spintronics and valleytronics.

Large Gap Two-Dimensional Topological Insulators with the Significant Rashba Effect in Ethynyl and Methyl Functionalized PbSn Monolayers

Two-dimensional (2D) topological insulators (TIs) have recently attracted a great deal of attention due to their nondissipation electron transmission, stable performance, and easy device integration. However, a primary obstacle to influencing 2D TIs is the small bandgap, which limits their room-temperature applications. Here, we adopted first-principles to predict inversion-asymmetric group IV monolayers, PbSn(C₂H)₂ and PbSn(CH₃)₂, to be quantum spin Hall (QSH) insulators with large topological gaps of 0.586 and 0.481 eV, respectively. The nontrivial band topologies, which can survive in a wide range of strain, are characterized by topological invariants Z₂, gapless edge states, and the Berry curvature. Another intriguing characteristic is the significant Rashba SOC effect which can also be tuned by feasible compressive and tensile strains. Meanwhile, the hexagonal boron nitride (h-BN) provides a suitable substrate for growth of these films without influencing their topological phases. These novel materials are expected to accelerate the development of advanced quantum devices.