Current-driven magnetization switching in a van der Waals ferromagnet Fe3GeTe2

The recent discovery of ferromagnetism in two-dimensional (2D) van der Waals (vdW) materials holds promises for novel spintronic devices with exceptional performances. However, in order to utilize 2D vdW magnets for building spintronic nanodevices such as magnetic memories, key challenges remain in terms of effectively switching the magnetization from one state to the other electrically. Here, we devise a bilayer structure of Fe3GeTe2/Pt, in which the magnetization of few-layered Fe3GeTe2 can be effectively switched by the spin-orbit torques (SOTs) originated from the current flowing in the Pt layer. The effective magnetic fields corresponding to the SOTs are further quantitatively characterized using harmonic measurements. Our demonstration of the SOT-driven magnetization switching in a 2D vdW magnet could pave the way for implementing low-dimensional materials in the next-generation spintronic applications

Anisotropic and isotropic friction on atomically flat surfaces

Friction has complex origins. However, most experimental methods only give total friction force and cannot separate friction forces from different origins. Thus, revealing underly friction mechanisms becomes a challenge. Here, we combine a two-dimensional friction force atomic force microscope method and a two-dimensional friction model to separate anisotropic and isotropic friction forces on atomically flat surfaces. We found that the friction force of most atomically flat surfaces is anisotropic, and the total force on the tip misaligns with the scan direction. The misalign angle can be used to calculate the ratio of anisotropic and isotropic friction components and the ratio of resistance forces from different elemental hopping combinations. The separation of anisotropic and isotropic components in the total friction force makes it possible to study friction forces from different origins, which is critical for further revealing the friction mechanism

Strongly distinct electrical response between circular and valley polarization in bilayer transition metal dichalcogenides

We introduce a physical model to describe the influence of a perpendicular electric field on circular polarization (CP) and valley polarization (VP) in bilayer transition metal dichalcogenides. Our results uncover that electric-field-dependent CP and VP are quite distinct from each other. The dependence of CP on the electric field harbors a W pattern and possesses the minimum when the potential energy difference between the two layers is equal to the strength of spin-orbit coupling. Such dependence of CP stems from the modulation of energy cost for interlayer hopping and spin-dependent layer polarization. In contrast, VP is strictly absent in primitive bilayers and increases monotonically with increasing strength of electric field, resulting from the continuous variation of valley magnetic moments and inversion-symmetry breaking. Our model elaborates well the recent experimental observations for which the origin is under debate. Moreover, we demonstrate that the manipulation of layer and valley pseudospin is fully tunable by perpendicular electric fields, paving the way for prospects in electrical control of exotic layer-valleytronics.Peer reviewe

2D proximate quantum spin liquid state in atomic-thin α-RuCl 3

Two-dimensional (2D) atomic crystals have made major inroads into condensed-matter physics and give rise to fascinating phenomena due to quantum confinement. Here we report the first Raman scattering study on phonon-magnetic scattering coupling, proximate quantum spin liquid ground state and collective fractionalized excitations in exfoliated α-RuCl 3 atomic layers. Our results uncover that 2D α-RuCl 3 could harbour the unusual magnetic continuum, serving as a hallmark of the 2D proximate quantum spin liquid state and frustrated magnetic interactions. More importantly, our work demonstrates that the unusual magnetic scattering, as compared with bulk, is more obvious in 2D α-RuCl 3, indicating that the frustrated magnetic interactions are enhanced strongly. Such unusual enhancement of frustrated magnetic interactions may be responsible for the gigantic phonon-magnetic scattering coupling of 2D α-RuCl 3 and play a key role in stabilizing the 2D proximate quantum spin liquid state. Our work establishes a firm basis for exploring and understanding the 2D proximate quantum spin liquid and fractionalized excitations based on the atomically thin α-RuCl 3.Peer reviewe

Enhancing and controlling valley magnetic response in MoS2/WS2 heterostructures by all-optical route

Van der Waals heterostructures of transition metal dichalcogenides with interlayer coupling offer an exotic platform to realize fascinating phenomena. Due to the type II band alignment of these heterostructures, electrons and holes are separated into different layers. The localized electrons induced doping in one layer, in principle, would lift the Fermi level to cross the spin-polarized upper conduction band and lead to strong manipulation of valley magnetic response. Here, we report the significantly enhanced valley Zeeman splitting and magnetic tuning of polarization for the direct optical transition of MoS2 in MoS2/WS2 heterostructures. Such strong enhancement of valley magnetic response in MoS2 stems from the change of the spin-valley degeneracy from 2 to 4 and strong many-body Coulomb interactions induced by ultrafast charge transfer. Moreover, the magnetic splitting can be tuned monotonically by laser power, providing an effective all-optical route towards engineering and manipulating of valleytronic devices and quantum-computation.Peer reviewe

Patterned Peeling 2D MoS₂ off the Substrate

The performance of two-dimensional (2D) MoS₂ devices depends largely on the quality of the MoS₂ itself. Existing fabrication process for 2D MoS₂ relies on lithography and etching. However, it is extremely difficult to achieve clean patterns without any contaminations or passivations. Here we report a peel-off pattering of MoS₂ films on substrates based on a proper interface engineering. The peel-off process utilizes the strong adhesion between gold and MoS₂ and removes the MoS₂ film contact with gold directly, leading to clean MoS₂ pattern generation without residuals. Significantly improved electrical performances including high mobility ∼17.1 ± 8.3 cm²/(V s) and on/off ratio ∼5.6 ± 3.6 × 10⁶ were achieved. Such clean fabrication technique paves a way to high quality MoS₂ devices for various electrical and optical applications

Giant valley coherence at room temperature in 3R WS2 with broken inversion symmetry

| openaire: EC/H2020/820423/EU//S2QUIPBreaking the space-time symmetries in materials can markedly influence their electronic and optical properties. In 3R-stacked transition metal dichalcogenides, the explicitly broken inversion symmetry enables valley-contrasting Berry curvature and quantization of electronic angular momentum, providing an unprecedented platform for valleytronics. Here, we study the valley coherence of 3R WS2 large single-crystal with thicknesses ranging from monolayer to octalayer at room temperature. Our measurements demonstrate that both A and B excitons possess robust and thickness-independent valley coherence. The valley coherence of direct A (B) excitons can reach 0.742 (0.653) with excitation conditions on resonance with it. Such giant and thickness-independent valley coherence of large single-crystal 3R WS2 at room temperature would provide a firm foundation for quantum manipulation of the valley degree of freedom and practical application of valleytronics.Peer reviewe

Strong and tunable interlayer coupling of infrared-active phonons to excitons in van der Waals heterostructures

Understanding and manipulating the quantum interlayer exciton-phonon coupling in van der Waals heterostructures, especially for infrared active phonons with electromagnetic fields, would set a foundation for realizing exotic quantum phenomena and optoelectronic applications. Here we report experimental observations of strong mutual interactions between infrared active phonons in hexagonal boron nitride (hBN) and excitons in WS2. Our results underscore that the infrared active A2u mode of hBN becomes Raman active with strong intensities in WS2/hBN heterostructures through resonant coupling to the B exciton of WS2. Moreover, we demonstrate that the activated A2u phonon of hBN can be tuned by the hBN thickness and harbors a striking anticorrelation intensity modulation, as compared with the optically silent B1g mode. Our observation of the interlayer exciton-infrared active phonon interactions and their evolution with hBN thickness provide a firm basis for engineering the hyperbolic exciton-phonon polaritons, chiral phonons and fascinating nanophotonics based on van der Waals heterostructures.Peer reviewe