The whorls of localized moments in chiral magnetic structures, such as skyrmions, lead to a quantized topological charge, which may make them useful as next-generation information bits. So far, the most reliable way to detect the existence of skyrmions is by using the topological Hall effect, which stems from electron scattering by the emergent magnetic field manifesting the topological charge. Here we employ two-dimensional magnets to establish a magneto-optical hallmark of skyrmions, which we call the topological Kerr effect, using the recently discovered ferromagnet CrVI6 as a material platform. The Kerr angle hysteresis loop of this non-centrosymmetric system exhibits two antisymmetric bumps that are absent in the centrosymmetric CrI3 and VI3. We develop a minimal model to further identify the bumps as direct manifestations of the topological charge, thereby providing a magneto-optical fingerprint of skyrmions with broader applicability.The ferromagnet CrVI6 serves as a material platform to demonstrate the topological Kerr effect in two-dimensional magnets. This can be used to identify skyrmions by magneto-optical means.

As a crucial concept in magnetism and spintronics, exchange bias (ExB) measures the asymmetry in the hysteresis loop of a pinned ferromagnet (FM)/antiferromagnet (AFM) interface. Previous studies are mainly focused on FM/AFM heterostructures composed of conventional bulk materials, whose complex interfaces prohibit precise control and full understanding of the phenomenon. Here, the enabling power of 2D magnets is exploited to demonstrate the emergence, non-aging, extendability, and rechargeability of ExB in van der Waals Fe3GeTe2 homostructures, upon moderate pressuring. The emergence of the ExB is attributed to a local stress-induced FM-to-AFM transition, as validated using first-principles calculations, and confirmed in magneto-optical Kerr effect and second harmonic generation measurements. It is also observed that, negligible ExB aging before the training effect suddenly takes place through avalanching, pronounced delay of the avalanche via timed pressure repetition (extendability), ExB recovery in the post-training sample upon refreshed pressuring (rechargeability), and demonstrate its versatile tunability. These striking findings offer unprecedented insights into the underlying principles of ExB and its training, with immense technological applications in sight. © 2022 Wiley-VCH GmbH.

Two-dimensional magnets have been discovered recently as a new class of quantum matter exhibiting a broad wealth of exotic phenomena, including notably various topological excitations rooted in emergent exchange couplings between the localized magnetic moments. By analyzing the anisotropies in the single-ion magnetization and two-body exchange couplings obtained from first-principles calculations, we reveal coexistence of both giant Dzyaloshinskii-Moriya interaction and strong anisotropic XXZ-type biquadratic coupling in a recently predicted monolayer CrMnI6 magnet. The former is induced by the spontaneous in-plane inversion symmetry breaking in the bipartite system, the latter is inherently tied to the distinct high-spin state of the Mn sublattice, while the large magnitudes of both stem from the significant spin-orbit coupling. Next, we use atomistic magnetics simulations to demonstrate the vital role of Dzyaloshinskii-Moriya interaction in harboring topological bimeronic excitations, and show that the biquadratic coupling favors a Berezinskii-Kosterlitz-Thouless-like transition as the system reduces its temperature from the paramagnetic phase. These findings substantially enrich our understanding of the microscopic couplings in 2D magnets, with appealing application potentials.

作为理想的清洁能源,氢气一直受到广泛关注。在通过析氢反应产氢的过程中,氢气的生成速率与氢原子在催化剂上的吸附能呈现著名的"火山型"依赖关系,其峰值附近的金属催化效率最高,但均为贵重金属。如何提高传统催化材料的活性,寻找相对廉价的新型催化剂,一直是该研究领域的核心问题。这里,我们通过第一性原理计算,围绕不同催化活性中心,探讨了几种基于二维层状材料的析氢反应的廉价催化剂材料。首先,研究了主要依赖于边缘活性中心的MoS2催化剂,通过揭示其边缘结构的原子尺度重构机制,试图理解MoS2边缘结构、边缘态与边缘化学活性的依赖关系。然后,通过不同调控手段改变材料物性,扩展到整个表面都有催化活性的体系。具体体系包括:1)借鉴限域催化的概念,通过石墨烯修饰不同金属表面,利用限域效应改变金属表面的催化活性,预言了单层石墨烯覆盖下的镍表面具有高催化活性,达到"点镍成金"的效果。2)利用拓扑绝缘体的鲁棒表面态来优化常规材料在析氢反应中的催化活性,预言了单层ZnSe/Bi2Se3异质结在表面悬挂键与拓扑表面态协同作用下催化活性达到最佳值。这些基于二维材料的研究为氢气的廉价制备以及其它工业上重要的化学过程提供了新思路。

As a crucial concept in magnetism and spintronics, exchange bias (ExB) measures the asymmetry in the hysteresis loop of a pinned ferromagnet (FM)/antiferromagnet (AFM) interface. Previous studies are mainly focused on FM/AFM heterostructures composed of conventional bulk materials, whose complex interfaces prohibit precise control and full understanding of the phenomenon. Here, the enabling power of 2D magnets is exploited to demonstrate the emergence, non-aging, extendability, and rechargeability of ExB in van der Waals Fe3GeTe2 homostructures, upon moderate pressuring. The emergence of the ExB is attributed to a local stress-induced FM-to-AFM transition, as validated using first-principles calculations, and confirmed in magneto-optical Kerr effect and second harmonic generation measurements. It is also observed that, negligible ExB aging before the training effect suddenly takes place through avalanching, pronounced delay of the avalanche via timed pressure repetition (extendability), ExB recovery in the post-training sample upon refreshed pressuring (rechargeability), and demonstrate its versatile tunability. These striking findings offer unprecedented insights into the underlying principles of ExB and its training, with immense technological applications in sight.

Stacking a graphene monolayer onto a three-dimensional topological insulator results in a van der Waals heterostructure that can serve as a versatile platform for rich emergent exotic physics. In particular, the Dirac electrons of the graphene and that of the two-dimensional surface states of the topological insulator hybridize effectively, leading to the emergence of two distinct electronic bands around the Fermi level, with respective Mexican-hat and bell shapes. In this paper, we explore the emergent collective plasmonic excitations in the representative heterostructure of graphene/Sb2Te3, combining first-principles calculations and low-energy effective Hamiltonian descriptions. We discover two different plasmon modes of the system, and our band-resolved analysis allows us to attribute the two modes to the intraband transitions within the Mexican-hat and bell-shaped bands, respectively. We also analyze the relative strengths and damping rates of the plasmon modes upon variations of the bias potential across the heterojunction, enabling potential experimental detections of those collective excitations. These findings provide alternate avenues for regulating the electronic states and plasmonic excitations in the hybridized Dirac electron systems for potential device applications.

High-quality stanene films have been actively pursued for realizing not only quantum spin Hall edge states without backscattering, but also intrinsic superconductivity, two central ingredients that may further endow the systems to host topological superconductivity. Yet to date, convincing evidence of topological edge states in stanene remains to be seen, let alone the coexistence of these two ingredients, owing to the bottleneck of growing high-quality stanene films. Here we fabricate one- to five-layer stanene films on the Bi(111) substrate and observe the robust edge states using scanning tunneling microscopy/spectroscopy. We also measure distinct superconducting gaps on different-layered stanene films. Our first-principles calculations further show that hydrogen passivation plays a decisive role as a surfactant in improving the quality of the stanene films, while the Bi substrate endows the films with nontrivial topology. The coexistence of nontrivial topology and intrinsic superconductivity renders the system a promising candidate to become the simplest topological superconductor based on a single-element system.

Two-dimensional transition metal dichalcogenides possessing superconductivity and strong spin-orbit coupling exhibit high in-plane upper critical fields due to Ising pairing. Yet to date, whether such systems can become topological Ising superconductors remains to be materialized. Here we show that monolayered NbSe2can be converted into Ising superconductors with nontrivial band topology via physical or chemical pressuring. Using first-principles calculations, we first demonstrate that a hydrostatic pressure higher than 2.5 GPa can induce a p-d band inversion, rendering nontrivial band topology to NbSe2. We then illustrate that Te-doping can function as chemical pressuring in inducing nontrivial topology in NbSe2-xTexwith x ≥ 0.8, due to a larger atomic radius and stronger spin-orbit coupling of Te. We also evaluate the upper critical fields within both approaches, confirming the enhanced Ising superconductivity nature, as experimentally observed. Our findings may prove to be instrumental in material realization of topological Ising superconductivity in two-dimensional systems. 2022 American Chemical Society. © 2022 American Chemical Society. All rights reserved.

Two-dimensional transition metal dichalcogenides possessing superconductivity and strong spin-orbit coupling exhibit high in-plane upper critical fields due to Ising pairing. Yet to date, whether such systems can become topological Ising superconductors remains to be materialized. Here we show that monolayered NbSe2 can be converted into Ising superconductors with nontrivial band topology via physical or chemical pressuring. Using first-principles calculations, we first demonstrate that a hydrostatic pressure higher than 2.5 GPa can induce a p-d band inversion, rendering nontrivial band topology to NbSe2. We then illustrate that Te-doping can function as chemical pressuring in inducing nontrivial topology in NbSe2-xTex with x >= 0.8, due to a larger atomic radius and stronger spin-orbit coupling of Te. We also evaluate the upper critical fields within both approaches, confirming the enhanced Ising superconductivity nature, as experimentally observed. Our findings may prove to be instrumental in material realization of topological Ising superconductivity in two-dimensional systems.

Materials realization of chiral topological superconductivity is a crucial condition for observing and manipulating Majorana fermions in condensed matter physics. Here we develop a tight-binding description of Pb3Bi/Ge(111), identified recently as an appealing candidate system for realizing chiral p-wave topological superconductivity [Nat. Phys. 15, 796 (2019)]. We first show that our phenomenological model can capture the two main features of the electronic band structures obtained from first-principles calculations, namely, the giant Rashba splitting and type-II van Hove singularity. Next, when the s-wave superconducting property of the parent Pb system is explicitly considered, we find the alloyed system can be tuned into a chiral topological superconductor with Chern number C = -2, resulting from the synergistic effect of a sufficiently strong Zeeman field and the inherently large Rashba spin-orbit coupling. The nontrivial topology with C = -2 is further shown to be detectable as two chiral Majorana edge modes propagating along the same direction of the system with proper boundaries. We finally discuss the physically realistic conditions to establish the predicted topological superconductivity and observe the corresponding Majorana edge modes, including the influence of the superconducting gap, Lande g factor, and critical magnetic field. The present study provides useful guides in searching for effective p-wave superconductivity and Majorana fermions in two-dimensional or related interfacial systems.