Abstract
Valleytronic materials, characterized by local extrema (valleys) in their bands, and topological insulators have separately attracted great interest recently. However, the interplay between valleytronic and topological properties in one single system, likely to enable important unexplored phenomena and applications, has been largely overlooked so far. Here, by combining a tight-binding model with first-principles calculations, we find the large-band-gap quantum spin Hall effects (QSHEs) and valley Hall effects appear simultaneously in the bismuth monolayers decorated with hydrogen/halogen elements, denoted as Bi2XY (X, Y = H, F, Cl, Br, or I). A staggered exchange field is introduced into the Bi2XY monolayers by transition-metal atom (Cr, Mo, or W) doping or LaFeO3 magnetic substrates, which together with the strong spin-orbit coupling of bismuth atoms generates a time-reversal-symmetry-broken QSHE and a huge valley splitting (up to 513 meV) in the system. With gate control, QSHE and anomalous charge, spin, valley Hall effects can be observed in the single system. These predicted multiple and exotic Hall effects, associated with various degrees of freedom of electrons, could enable applications of the functionalized bismuth monolayers in electronics, spintronics, and valleytronics.
Highlights
Tailoring valley degrees of freedom offers fascinating opportunities to realize novel phenomena and emerging applications, often referred to as valleytronics.[1–4] While valley effects have been studied for decades in materials such as silicon,[5] diamond,[6] AlAs,[7] and graphene,[8–11] despite the effort to emulate the better known manipulation of spin and spintronic applications,[12] the related success has been modest.[2]
The resurgence of interest in valley effects was recently spurred by the discovery of monolayer (ML) transitionmetal dichalcogenides (TMDs) with broken inversion symmetry and strong spin-orbit coupling (SOC).[14–22]
A hallmark of ML TMDs is their valley-spin coupling, which leads to a valley-dependent helicity of optical transitions[23–25] as well as important implications for transport, such as the discovery of the valley Hall effect (VHE).[4]
Summary
Tailoring valley degrees of freedom offers fascinating opportunities to realize novel phenomena and emerging applications, often referred to as valleytronics.[1–4] While valley effects have been studied for decades in materials such as silicon,[5] diamond,[6] AlAs,[7] and graphene,[8–11] despite the effort to emulate the better known manipulation of spin and spintronic applications,[12] the related success has been modest.[2]. With the SOC, large nontrivial band gaps from 0.891 to 1.256 eV (see Table S1 and Fig. S3 in Supplementary Information) are opened in the two valleys, giving rise to quantum spin Hall effect and valley Hall effect.
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