Quantum spin Hall insulators and quantum valley Hall insulators of BiX/SbX (X = H, F, Cl, and Br) monolayers with a record bulk band gap (1402.2399v3)

Abstract: Large bulk band gap is critical for application of the quantum spin Hall (QSH) insulator or two dimensional (2D) topological insulator (TI) in spintronic device operating at room temperature (RT). Based on the first-principles calculations, here we predict a group of 2D topological insulators BiX/SbX (X = H, F, Cl, and Br) monolayers with extraordinarily large bulk gaps from 0.32 to a record value of 1.08 eV. These giant-gaps are entirely due to the result of strong spin-orbit interaction related to px and py orbitals of Bi/Sb atoms around the two valley K and K' of honeycomb lattice, which is different significantly from the one consisted of pz orbital just like in graphene/silicene. The topological characteristic of BiX/SbX monolayers is confirmed by the calculated nontrivial Z2 index and an explicit construction of the low energy effective Hamiltonian in these systems. We show that the honeycomb structures of BiX monolayers remain stable even at a temperature of 600 K. These features make the giant-gap TIs BiX/SbX monolayers an ideal platform to realize many exotic phenomena and fabricate new quantum devices operating at RT. Furthermore, biased BiX/SbX monolayers become a quantum valley Hall insulator, showing valley-selective circular dichroism.

• The paper demonstrates that BiX/SbX monolayers exhibit robust quantum spin Hall phases with record bulk band gaps up to 1.08 eV, even at 600 K.
• Methodologies include first-principles calculations, VASP and WIEN2K geometry optimization, and ab initio MD simulations to ensure thermal and structural stability.
• The findings imply that tuning spin-orbit and valley coupling can advance room-temperature spintronics and valleytronics in 2D topological materials.

Quantum Spin Hall and Quantum Valley Hall Insulators in BiX/SbX Monolayers

The paper on Quantum Spin Hall (QSH) and Quantum Valley Hall (QVH) insulators in BiX/SbX (X = H, F, Cl, and Br) monolayers introduces a novel class of topological insulators with record-setting bulk band gaps, significantly advancing the application potential of 2D topological materials in spintronics and quantum devices. Utilizing first-principles calculations, this research identifies BiX/SbX monolayers capable of maintaining a robust topological phase with an exceptionally wide band gap, maintained even at a temperature of up to 600 K. This work is of notable interest due to the strong spin-orbit coupling (SOC) effects observed, primarily driven by the px and py orbitals in Bi/Sb atoms, as opposed to the traditionally studied pz orbitals in graphene and silicene.

Key Findings and Methodologies

The authors have utilized geometry optimization through the VASP and WIEN2K packages, employing the projector augmented wave (PAW) pseudopotential method for an accurate prediction of the electronic properties of these monolayers. Calculations were rigorously conducted with SOC inclusion to identify the nontrivial Z₂ index and construct the low-energy effective Hamiltonian (LEEH).

Key structural results indicate that BiX monolayers (X = H, F, Cl, and Br) exhibit lattice and buckling characteristics aligning with their electronegativity and covalent bond radii. Thermal and structural stabilities were confirmed by ab initio molecular dynamics (MD) simulations across different temperature settings, ensuring applicability under realistic conditions.

Quantum mechanical modeling showed that SOC significantly lifts the degeneracy at the Dirac points, resulting in band gaps ranging from 0.74 to 1.08 eV in BiX and 0.32 to 0.41 eV in SbX monolayers. The Z₂ index consistently confirmed the nontrivial topological nature, making them suitable candidates as QSH insulators with robust helical edge states.

Implications and Future Directions

The manifestation of large-band-gap QSH effects in BiX/SbX monolayers enhances their potential for room-temperature applications. The demonstrated quantum phase transitions into QVH insulators by breaking inversion symmetry further augments their functional flexibility by introducing valley-selective circular dichroism. These features portend new capabilities in valleytronics and spintronics, notably enhancing the performance attributes of transistors and other electronic devices.

Furthermore, this work highlights the potential for chemical functionalization as a method to tune band gaps and other electronic properties in 2D materials. The findings on the strong spin-valley coupling, distinctly pronounced due to large SOC, suggest an exciting frontier for manipulating charge carriers in novel ways, with ramifications for future quantum computing architectures.

The paper also points toward experimental feasibility, recommending preparation methods analogous to those used for graphane synthesis. Given the robustness of these monolayers against deformation and oxidation, and their potential synthesis using methods such as exfoliation or hydrogen plasma treatment, experimental synthesis and characterization appear reachable aims. Continued exploration in this domain may lead to practical realization of the theoretical predictions, driving further advancements in high-temperature spintronic devices and topological quantum computing technologies.

In conclusion, the work not only underscores critical insights into the quantum mechanical properties of BiX/SbX monolayers but also propels forward the theoretical and practical paper of robust topological phases, stimulating substantive experimental and theoretical initiatives in the field of two-dimensional topological insulators.