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van der Waals (vdW) magnets have rapidly emerged as a fertile playground for fundamental physics and exciting applications. Despite the impressive developments over the past few years, technical limitations pose a severe challenge to many other potential breakthroughs. High on the list is the lack of suitable experimental tools for studying spin dynamics on atomically thin samples. Here, Raman scattering techniques are employed to directly observe the low-lying magnon (∼1 meV) even in bilayer NiPS3. The advantage is that it offers excellent energy resolutions far better on low-energy sides than most inelastic neutron spectrometers can offer. More importantly, with appropriate theoretical analysis, the polarization dependence of the Raman scattering by those low-lying magnons also provides otherwise hidden information on the dominant spin-exchange scattering paths for different magnons. By comparing with high-resolution inelastic neutron scattering data, these low-energy Raman modes are confirmed to be indeed of magnon origin. Because of the different scattering mechanisms involved in inelastic neutron and Raman scattering, this information is fundamental in pinning down the final spin Hamiltonian. This work demonstrates the capability of Raman spectroscopy to probe the genuine two-dimensional spin dynamics in atomically thin vdW magnets, which can provide insights that are obscured in bulk spin dynamics.

The impact of delaying atrial fibrillation catheter ablation (AFCA) for antiarrhythmic drug (AAD) management on the disease course remains unclear. This study investigated AFCA rhythm outcomes based on the diagnosis‐to‐ablation time (DAT) and AAD responsiveness in participants with persistent AF (PeAF). We included data from 1038 AAD‐resistant PeAF participants, all of whom had a clear time point for AF diagnosis, especially PeAF at diagnosis time, and had undergone an AFCA for the first time. Participants who experienced recurrences of paroxysmal type on AAD therapy were analyzed as a cohort of AAD‐partial responders; those maintaining PeAF on AAD were AAD‐non‐responders. We determined the DAT cutoff for best discriminating long‐term rhythm outcomes using a maximum log‐likelihood estimation method based on the Cox proportional hazard regression model. Of the participants (79.8% male; median age 61), 806 (77.6%) were AAD‐non‐responders. AAD‐non‐responders had a higher body mass index and a larger left atrial diameter than AAD‐partial‐responders. They also had a higher incidence of AF recurrence after AFCA (adjusted hazard ratio 1.75, 95% confidence interval 1.33–2.30; log‐rank p< .001) compared to AAD‐partial‐responders. The maximum log‐likelihood estimation showed bimodal cutoffs at 22 and 40 months. The optimal DAT cutoff rhythm outcome was 22 months, which discriminated better in the AAD‐partial‐responders than in the AAD‐non‐responders. Both DAT and AAD responsiveness influenced AFCA rhythm outcomes. Delaying AFCA to a DAT of longer than 22 months was inadvisable, particularly in the participants in whom PeAF was changed to paroxysmal AF during AAD therapy. AAD‐non‐responders had a higher incidence of AF recurrence after AFCA compared to AAD‐partial‐responders. The maximum log‐likelihood estimation showed bimodal cutoffs at 22 and 40 months. The optimal DAT cutoff rhythm outcome was 22 months, which discriminated better in the AAD‐partial‐responders than in the AAD‐non‐responders.

Practical application of lithium- and manganese-rich layered oxide cathodes has been hindered despite their high performance and low cost owing to high gas evolution accompanying capacity loss even in a conservative voltage window. Here, we control the surface structure and primary particle size of lithium- and manganese-rich layered oxide cathodes not only to enhance the electrochemical performance but also to reduce gas evolution. Sulfur-coated Fm3¯m/R3¯m double reduced surface layers and Mo doping dramatically reduce gas evolution, which entails the improvement of electrochemical performance. With the optimization, we prove that it is competitive enough to conventional high-nickel cathodes in the aspects of gas evolution as well as electrochemical performance in the conservative voltage window of 2.5–4.4 V. Our findings provide invaluable insights on the improvement of electrochemical performance and gas evolution properties in lithium- and manganese-rich layered oxide cathodes.

As dynamic random access memory (DRAM) technologies continue to be downscaled, the partial isolation type buried channel array transistor (Pi-BCAT) structure has emerged as an innovative solution for the increasing challenges caused by leakage current adjacent to passing word lines (PWLs). This study reveals that the Pi-BCAT reduces leakage currents by 30% when compared to conventional BCAT structures. Our comprehensive simulations demonstrate that Pi-BCAT is resistant to temperature-induced leakage variations, confirming its significance in promoting consistent device performance and power management. The Pi-BCAT structure is predicted to be crucial in the advancement of DRAM reliability and efficiency, hence initiating further advancements in semiconductor technology.

The unique discovery of the magnetic exciton in van der Waals antiferromagnet NiPS3arises between two quantum many-body states of a Zhang-Rice singlet excited state and a Zhang-Rice triplet ground state. Simultaneously, the spectral width of photoluminescence originating from this exciton is exceedingly narrow as 0.4 meV. These extraordinary properties, including the extreme coherence of the magnetic exciton in NiPS3, beg many questions. We studied doping effects using Ni1–xCdxPS3using two experimental techniques and theoretical studies. Our experimental results show that the magnetic exciton is drastically suppressed upon a few % Cd doping. All this happens while the width of the exciton only gradually increases and the antiferromagnetic ground state is robust. These results highlight the lattice uniformity’s hidden importance as a prerequisite for coherent magnetic exciton. Finally, an exciting scenario emerges: the broken charge transfer forbids the otherwise uniform formation of the coherent magnetic exciton in (Ni,Cd)PS3.

The application of cytocompatible hydrogels supporting extensive cellular activities to three-dimensional (3D) bioprinting is crucial for recreating complex physiological environments with high biomimicry. However, the poor printability and tunability of such natural hydrogels diminish the versatility and resolution of bioprinters. In this study, we propose a novel approach for the hybrid biofabrication of complex and heterogeneous 3D constructs using low-viscosity bioinks. Poly(lactic acid) (PLA) filament is extruded by fused deposition modeling on a micromesh to create PLA-framed micromesh substrates onto which fibrinogen is printed by microextrusion bioprinting. The micromesh supports the printed hydrogel with a capillary pinning effect to enable high-resolution bioprinting. Accordingly, the micromesh–bioink layers are aligned and stacked to form volumetric constructs. This approach, called the 3D micromesh–bioink overlaid structure and interlocked culture (3D MOSAIC) platform, enables the fabrication of complicated and multimaterial 3D structures, including overhangs and voids. Endothelial cells cultured under vasculogenic conditions in the platform self-organize within the biologically functional hydrogel to form vascular networks, and cancer cell migration can be observed across the layers. The multidisciplinary 3D MOSAIC platform is an important step toward the biofabrication of complex constructs with high biological and structural significance and functionality.

ObjectiveThe risk of recurrence after atrial fibrillation (AF) catheter ablation (AFCA) is higher in women than in men. However, it is unknown whether a sex difference exists in antiarrhythmic drug (AAD) responsiveness among patients with recurrence.MethodsAmong 2999 consecutive patients (26.5% women, 58.3±10.9 years old, 68.1% paroxysmal AF) who underwent de novo AFCA, we compared and evaluated the sex differences in rhythm outcome in 1094 patients with recurrence and in 788 patients who subsequently underwent rhythm control with AAD.ResultsDuring a follow-up of 48.2±34.9 months, 1094 patients (36.5%) had AF recurrence after AFCA, and 508 of 788 patients (64.5%) had AF recurrence under AAD. Although the rhythm outcome of a de novo AFCA was worse (log-rank p=0.041, HR 1.28, 95% CI 1.02 to 1.59), p=0.031) in women, AAD response after postprocedural recurrences was better in women than in men (log-rank p=0.003, HR 0.75, 95% CI 0.59 to 0.95, p=0.022), especially in women older than 60 years old (log-rank p=0.003). In 249 patients who underwent repeat procedure after AAD use, the pulmonary vein (PV) reconnection rate (62.7% vs 76.8%, p=0.048) was lower in women than in men but not the existence of extra-PV trigger (37.8% vs 25.4%, p=0.169).ConclusionsAlthough women showed worse rhythm outcomes than men after AFCA, the post-AFCA AAD response was better in elderly women than in men.Trial registration numberNCT02138695.

The Kitaev model, a honeycomb network of spins with bond-dependent anisotropic interactions, is a rare example of a system with a quantum spin liquid ground state. Although most Kitaev model candidate materials eventually order magnetically due to additional non-Kitaev interactions, their bond-dependent anisotropy manifests in unusual spin dynamics. Recent research suggests that bond-dependent anisotropy can stabilize exotic magnetic phases on the geometrically frustrated triangular lattice. Unfortunately, few materials have been identified with simultaneous geometric frustration and bond-dependent anisotropy. Here, we report a frustrated triangular lattice with bond-dependent anisotropy in the cobalt-based van der Waals antiferromagnet CoI2. Momentum and energy-resolved inelastic neutron scattering measurements show substantial magnon decay and level repulsion. A thorough examination of excitations in both the paramagnetic and magnetically ordered states demonstrates that the bond-dependent anisotropy is the origin of the spiral order and the magnon decay found in CoI2. Our results provide the basis for future studies of the interplay between Kitaev magnetism and geometric frustration.

In this article, we propose an analysis of the usage of a partial isolation type buried channel array transistor (Pi-BCAT). Compared with other structures, the conventional BCAT exhibits improved characteristics in the row hammer effect (RHE) because of its shallow drain/body (D/B) junction. Nevertheless, it remains affected by the RHE and should be mitigated because it is directly related to the reliability of dynamic random access memory (DRAM) applications. The proposed device exhibits a 50% improvement in the RHE and reduces leakage current ( ${I}_{{\mathrm {OFF}}}$ ) to one-third the level of conventional BCATs while also minimizing the ON -current ( ${I}_{{\mathrm {ON}}}$ ) reduction. Moreover, to efficiently compare RHE, we compare $\Delta {V}_{\text {SN}}$ by RHE and $\Delta {V}_{\text {SN}}$ based on the gate-induced drain leakage (GIDL) according to bias conditions and the device’s parameters. Finally, we optimize the parameter values of the buried insulator by considering electrical characteristics and the RHE.

In response to the increasing demand for spheroid‐based cancer research, the importance of developing integrated platforms that can simultaneously facilitate high‐throughput spheroid production and multiplexed analysis is emphasized. In addition, the understanding of how the size and cellular composition of tumors directly influence their internal structures and functionalities underlines the critical need to produce spheroids of diverse sizes and compositions on a large scale. To address this rising demand, this work presents a configurable and linkable in vitro three‐dimensional (3D) cell culture kit (CLiCK) for spheroids, termed CLiCK‐Spheroid. This platform consists of three primary components: a hanging drop microarray (HDMA), a concave pillar microarray (CPMA), and gradient blocks. The HDMA alone produces a homogeneous spheroid array, while its combination with the gradient block enables one‐step generation of a size‐gradient spheroid array. Using the size‐gradient spheroid arrays, the occurrence of necrotic cores based on spheroid size is demonstrated. Additionally, spheroids in a single batch can be conveniently compartmentalized and regrouped using a CPMA, enhancing the versatility of spheroid arrays and enabling multiplexed drug treatments. By combining the different assembly methods, this work has achieved high‐throughput production of cell composition‐gradient spheroid arrays, with noticeable variations in morphology and vascularization based on cell compositions. A configurable and linkable in vitro 3D cell culture kit for spheroids (CLiCK‐Spheroid) enables high‐throughput production of homogeneous, size‐gradient, and cell composition‐gradient spheroid arrays using a hanging drop microarray and gradient block. The generated spheroid array can be retrieved and transferred on demand using a concave pillar microarray. Multiplexed, sequential drug treatment and spheroid penetration and vascularization assay are demonstrated.

Composition‐Gradient Spheroid Array The reconfigurable hanging drop microarray platform enables high‐throughput production of a cell composition‐gradient spheroid array using a hanging drop microarray and a gradient block. The resulting spheroid array can be effectively retrieved and transferred for subsequent analysis using a concave pillar microarray. More details can be found in article 2400501by Je‐Kyun Park and co‐workers.

Current organ‐on‐a‐chip technologies confront limitations in effectively recapitulating the intricate in vivo microenvironments and accommodating diverse experimental conditions on a single device. Here, a novel approach for constructing a multi‐composition tumor array on a single microfluidic device, mimicking complex transport phenomena within tumor microenvironments (TMEs) and allowing for simultaneous evaluation of drug efficacy across 12 distinct conditions is presented. The TME array formed by bioprinting on a microfluidic substrate consists of 36 individual TME models, each characterized by one of three different compositions and tested under four varying drug concentrations. Notably, the TME model exhibits precise compartmentalization, fostering the development of self‐organized vascular endothelial barriers surrounding breast cancer spheroids affecting substance transport. Multivariable screening and analysis of diverse conditions, including model complexity, replicates, and drug concentrations, within a single microfluidic platform, highlight the synergistic potential of integrating bioprinting with microfluidics to evaluate drug responses across diverse TME conditions comprehensively. A novel approach is reported by integrating bioprinting and microfluidic technologies to create a multi‐composition tumor array that mimics the complex transport in the tumor microenvironment (TME). This approach enables the self‐organized formation of a vascular barrier around tumor spheroids and comprehensively evaluates drug efficacy across different conditions of TME models.

ReS2is a group-VII chalcogenide with a crystal structure that has inversion symmetry only. Due to the low symmetry, it has in-plane anisotropy, and the two vertical orientations are not equivalent. The in-plane anisotropy leads to optical birefringence that can be observed by using polarized optical microscopy. We found ReS2crystals with domains of inequivalent vertical orientations but with the same Re-chain directions. Polarized Raman spectroscopy was used to determine the vertical orientations and the b-axis (Re-chain) directions of the domains, and high-resolution scanning transmission electron microscopy measurements confirmed that the Re-chain directions of the two types of the neighboring domains are exactly parallel. From polarized optical reflectivity measurements of the two types of domains, we found that the optical slow axis is not along the b-axis as previously believed but is tilted by ∼2.4° from the b-axis of the crystal. This offset makes the two neighboring domains with parallel Re-chains optically inequivalent and enables one to observe optical contrast between the two types of domains in polarized optical microscopy. We propose a quick and easy method to determine the crystallographic orientations of such domains by using polarized optical microscopy only.

Schizophrenia (SPR) is the most devastating mental illness that causes severe deterioration in social and occupational functioning, but, the etiology remains unknown. The objective of this study is to explore the genetic underpinnings of novelty seeking behavior in schizophrenic family within the Korean population. By conducting a family-based genome-wide association study, we aim to identify potential genetic markers and variations associated with novelty seeking traits in the context of SPR. We have recruited 27 probands (with SPR) with their parents and siblings whenever possible. DNA was extracted from blood sampling of 58 individuals in 27 families and analyzed in an Illumina core exome single nucleotide polymorphism (SNP) array. A family-based association test (qFAM) was used to derive SNP association values across all chromosomes. Although none of the final 800,000 SNPs reached the genome-wide significant threshold of 8.45 × 10−7, the most significant 4 SNPs were within the 10–5to 10–7. This study identifies genetic associations between novelty seeking behavior and SPR within families. RAPGEF5emerges as a significant gene, along with other neuropsychiatric-related genes. Noteworthy genes like DRD4and COMTdid not show associations, possibly due to the focus on schizophrenic family. While shedding light on this complex relationship, larger studies are needed for robust conclusions and deeper mechanistic insights.

•The purpose of this study is to investigate the effect of low cortical thickness or volume of medial temporal sub-regions on risk of incident psychosis in patients with AD•We found a low cortical thickness or volume of medial temporal area was associated with a higher risk of incident psychosis in patients with AD; low cortical thickness of left entorhinal cortex (HR [95% CI]: 4.291 [1.196–15.384]), and low cortical volume of right hippocampus (HR [95% CI]: 2.680 [1.003–1.196]).•Our study demonstrates that low cortical thickness or volume of medial temporal sub-regions is a risk factor for incident psychosis in patients with AD. A careful assessment of medial temporal sub-regional thickness or volume in AD may improve early detection and intervention of psychosis in AD.

Strong periodic driving with light offers the potential to coherently manipulate the properties of quantum materials on ultrafast timescales. Recently, strategies have emerged to drastically alter electronic and magnetic properties by optically inducing non-trivial band topologies1–6, emergent spin interactions7–11and even superconductivity12. However, the prospects and methods of coherently engineering optical properties on demand are far less understood13. Here we demonstrate coherent control and giant modulation of optical nonlinearity in a van der Waals layered magnetic insulator, manganese phosphorus trisulfide (MnPS3). By driving far off-resonance from the lowest on-site manganese d–dtransition, we observe a coherent on–off switching of its optical second harmonic generation efficiency on the timescale of 100 femtoseconds with no measurable dissipation. At driving electric fields of the order of 109volts per metre, the on–off ratio exceeds 10, which is limited only by the sample damage threshold. Floquet theory calculations14based on a single-ion model of MnPS3are able to reproduce the measured driving field amplitude and polarization dependence of the effect. Our approach can be applied to a broad range of insulating materials and could lead to dynamically designed nonlinear optical elements.

The highly complex central nervous systems of mammals are often studied using three-dimensional (3D) in vitro primary neuronal cultures. A coupled confocal microscopy and immunofluorescence labeling are widely utilized for visualizing the 3D structures of neurons. However, this requires fixation of the neurons and is not suitable for monitoring an identical sample at multiple time points. Thus, we propose a label-free monitoring method for 3D neuronal growth based on refractive index tomograms obtained by optical diffraction tomography. The 3D morphology of the neurons was clearly visualized, and the developmental processes of neurite outgrowth in 3D spaces were analyzed for individual neurons.

Ferromagnetism in two-dimensional materials presents a promising platform for the development of ultrathin spintronic devices with advanced functionalities. Recently discovered ferromagnetic van der Waals crystals such as CrI3, readily isolated two-dimensional crystals, are highly tunable through external fields or structural modifications. However, there remains a challenge because of material instability under air exposure. Here, we report the observation of an air-stable and layer-dependent ferromagnetic (FM) van der Waals crystal, CrPS4, using magneto-optic Kerr effect microscopy. In contrast to the antiferromagnetic (AFM) bulk, the FM out-of-plane spin orientation is found in the monolayer crystal. Furthermore, alternating AFM and FM properties observed in even and odd layers suggest robust antiferromagnetic exchange interactions between layers. The observed ferromagnetism in these crystals remains resilient even after the air exposure of about a day, providing possibilities for the practical applications of van der Waals spintronics.

Realizing a state of matter in two dimensions has repeatedly proven a novel route of discovering new physical phenomena. Van der Waals (vdW) materials have been at the center of these now extensive research activities. They offer a natural way of producing a monolayer of matter simply by mechanical exfoliation. This work demonstrates that the possible multiferroic state with coexisting antiferromagnetic and ferroelectric orders persists down to the bilayer flake of NiI2. By exploiting the optical second-harmonic generation technique, both magnitude and direction of the ferroelectric order, arising from the cycloidal spin order, are successfully traced. The possible multiferroic state’s transition temperature decreases from 58 K for the bulk to about 20 K for the bilayer. Our observation will spur extensive efforts to demonstrate multifunctionality in vdW materials, which have been tried mostly by using heterostructures of singly ferroic ones until now.

The family of group IV–VI monochalcogenides has an atomically puckered layered structure, and their atomic bond configuration suggests the possibility for the realization of various polymorphs. Here, we report the synthesis of the first hexagonal polymorph from the family of group IV–VI monochalcogenides, which is conventionally orthorhombic. Recently predicted four-atomic-thick hexagonal GeSe, so-called γ-GeSe, is synthesized and clearly identified by complementary structural characterizations, including elemental analysis, electron diffraction, high-resolution transmission electron microscopy imaging, and polarized Raman spectroscopy. The electrical and optical measurements indicate that synthesized γ-GeSe exhibits high electrical conductivity of 3 × 105S/m, which is comparable to those of other two-dimensional layered semimetallic crystals. Moreover, γ-GeSe can be directly grown on h-BN substrates, demonstrating a bottom-up approach for constructing vertical van der Waals heterostructures incorporating γ-GeSe. The newly identified crystal symmetry of γ-GeSe warrants further studies on various physical properties of γ-GeSe.

ObjectiveThe risk of procedure-related complications and rhythm outcomes differ between men and women after atrial fibrillation catheter ablation (AFCA). We evaluated whether consistent sex differences existed in mapping and rhythm outcomes in repeat ablation procedures.MethodsAmong 3282 patients in the registry, we analysed 443 consecutive patients (24.6% female, 58.5±10.3 years old, 61.5% with paroxysmal atrial fibrillation) who underwent a second AFCA. We compared the clinical factors, mapping, left atrial (LA) pressure, complications and long-term clinical recurrences after propensity score matching.ResultsLA volume index (43.1±18.6 vs 35.8±11.6 mL/m2, p<0.001) was higher, but LA dimension (40.0±6.8 vs 41.6±6.3 mm, p=0.018), LA voltage (0.94±0.55 vs 1.20±0.68 mV, p=0.002) and pericardial fat volume (89.5±43.1 vs 122.1±53.9 cm3, p<0.001) were lower in women with repeat ablation than in their male counterparts. Pulmonary vein (PV) reconnections were lower (58.7% vs 74.9%, p=0.001), but the proportion of extra-PV triggers (27.5% vs 17.0%, p=0.026) and elevated LA pulse pressures (79.7% vs 63.7%, p=0.019) was significantly higher in women than in men. There was no significant sex difference in the rate of procedure-related complications (4.6% vs 4.2%, p=0.791). During a 31-month (8–60) median follow-up, clinical recurrences were significantly higher in women after both the de novo procedure (log-rank p=0.039, antiarrhythmic drug (AAD)-free log-rank p<0.001) and the second procedure (log-rank p=0.006, AAD-free log-rank p=0.093). Female sex (HR 1.51, 95% CI 1.06 to 2.15, p=0.023), non-paroxysmal atrial fibrillation (HR 1.78, 95% CI 1.30 to 2.34, p<0.010) and extra-PV triggers (HR 1.88, 95% CI 1.28 to 2.75, p=0.001) were independently associated with clinical recurrences after repeat procedures.ConclusionsDuring repeat AFCA procedures, PV reconnections were lower in women than in men, and the existence of extra-PV triggers and an LA pressure elevation were more significant, which resulted in poor rhythm outcomes.Trial registration numberNCT02138695.

Fabrication of a 3D in vitro model that mimics the artery takes an important role in understanding pathological cell behaviors and mechanisms of vascular diseases by proposing an advanced model that can recapitulate a native vessel condition in a controlled manner. Because a model geometry and the structure of cells are significant for the recapitulation of the hemodynamics of arterial and cell functions, it is necessary to mimic geometries and to induce the proper morphology and orientation of the cells when fabricating a model. In this study, smooth muscle cells (SMCs) and endothelial cells (ECs), which were the main elements in the arterial wall, were cocultured in a multichannel device connected with polydimethylsiloxane (PDMS) fluidic chamber modules to parallelly fabricate a pefusable 3D in vitro human artery-mimicking multichannel system. In the coculture model, a circular PDMS channel with a wrinkled-surface guided directionality and contractile morphology to SMCs, and media perfusion induced directionality to a confluent EC layer as in vivo. Protein markers of cells and synthesized extracellular matrices were demonstrated. Because multichannels were connected to a microfluidic module in a device, it was possible to easily control the microenvironmental conditions and to fabricate coculture models in parallel with a single flow system. Coculture models that can be tuned in designs such as diameter, wall shear stress, and geometry of artery disease were constructed by 3D-printed molds to recapitulate various cellular microenvironments and to model vessels effectively. Finally, the effect of wall shear stress on cells was compared using a device with four different degrees of stenosis channels and investigated in parallel.

This study explored the potential mechanisms of long-term recurrence (LTR) of atrial fibrillation (AF) 3 years after AF catheter ablation (AFCA).

An exciton is the bosonic quasiparticle of electron–hole pairs bound by the Coulomb interaction1. Bose–Einstein condensation of this exciton state has long been the subject of speculation in various model systems2,3, and examples have been found more recently in optical lattices and two-dimensional materials4–9. Unlike these conventional excitons formed from extended Bloch states4–9, excitonic bound states from intrinsically many-body localized states are rare. Here we show that a spin–orbit-entangled exciton state appears below the Néel temperature of 150 kelvin in NiPS3, an antiferromagnetic van der Waals material. It arises intrinsically from the archetypal many-body states of the Zhang–Rice singlet10,11, and reaches a coherent state assisted by the antiferromagnetic order. Using configuration-interaction theory, we determine the origin of the coherent excitonic excitation to be a transition from a Zhang–Rice triplet to a Zhang–Rice singlet. We combine three spectroscopic tools—resonant inelastic X-ray scattering, photoluminescence and optical absorption—to characterize the exciton and to demonstrate an extremely narrow excitonic linewidth below 50 kelvin. The discovery of the spin–orbit-entangled exciton in antiferromagnetic NiPS3introduces van der Waals magnets as a platform to study coherent many-body excitons.

The recent discovery of magnetic van der Waals (vdW) materials provides a platform to answer fundamental questions on the two-dimensional (2D) limit of magnetic phenomena and applications. An important question in magnetism is the ultimate limit of the antiferromagnetic layer thickness in ferromagnetic (FM)/antiferromagnetic (AFM) heterostructures to observe the exchange bias (EB) effect, of which origin has been subject to a long-standing debate. Here, we report that the EB effect is maintained down to the atomic bilayer of AFM in the FM (Fe3GeTe2)/AFM (CrPS4) vdW heterostructure, but it vanishes at the single-layer limit. Given that CrPS4is of A-type AFM and, thus, the bilayer is the smallest unit to form an AFM, this result clearly demonstrates the 2D limit of EB; only one unit of AFM ordering is sufficient for a finite EB effect. Moreover, the semiconducting property of AFM CrPS4allows us to electrically control the exchange bias, providing an energy-efficient knob for spintronic devices.

Polarized Raman spectroscopy on few-layer ReS2and ReSe2was carried out to determine the crystallographic orientations. Since monolayer ReX2(X = S or Se) has a distorted trigonal structure with only an inversion center, there is in-plane anisotropy and the two faces of a monolayer crystal are not equivalent. Since many physical properties vary sensitively depending on the crystallographic orientation, it is important to develop a reliable method to determine the crystal axes of ReX2. By comparing the relative polarization dependences of some representative Raman modes measured with three different excitation laser energies with high-resolution scanning transmission electron microscopy, we established a reliable procedure to determine all three principal directions of few-layer ReX2, including a way to distinguish the two types of faces: a 2.41 eV laser for ReS2or a 1.96 eV laser for ReSe2should be chosen as the excitation source of polarized Raman measurements; then the relative directions of the maximum intensity polarization of the Raman modes at 151 and 212 cm−1(124 and 161 cm−1) of ReS2(ReSe2) can be used to determine the face types and the Re-chain direction unambiguously.