Control of structure and spin texture in the van der Waals layered magnet CrSBr

Magnetic Field Tunable Polaritons in the Ultrastrong Coupling Regime in CrSBr

Collective excitations of bound electron-hole pairs, i.e., excitons, are ubiquitous in condensed matter systems, and it has been shown that they can strongly couple to other degrees of freedom, such as spin, orbital, and lattice. Among van der Waals materials with pronounced excitonic responses, CrSBr has attracted significant interest due to a very large energy shift of its fundamental bright exciton (at 1.36 eV) under an external magnetic field. This effect has been associated with an increased interlayer electronic hopping when the magnetic order is switched from antiferromagnetic to ferromagnetic by an external magnetic field, enabling its optical detection. In this work, we report the observation of a second bright excitonic resonance (at 1.76 eV), displaying a 5-fold enhancement of the magnetically induced energy shift to ∼100 meV, which we associate to a decreased spatial localization and increased interband nature compared to the fundamental exciton. Moreover, we show how the light-matter interaction reaches the ultrastrong regime where this exciton hybridizes with the cavity modes of photons confined to CrSBr flakes, forming polaritons with a Rabi splitting of ℏΩR ≃ 372 meV, of the same order of magnitude as the one reported for the first exciton. These results expand the understanding of the relationship between the optical response and band structure of CrSBr and clarify the essential ingredients for optimizing magneto-electronic coupling for applications.

Enhanced water decontamination via photogenerated electron delocalization of π → π* and D-π-A synergistically

Mixed-dimensional van der Waals heterojunctions (MD-vdWhs), known for exceptional electron transfer and charge separation capabilities, remain underexplored in photocatalysis. In this study, we leveraged the synergistic effect of intermolecular π → π* and D-π-A dual channels to fabricate novel MD-vdWhs. Owing to the synergistic effect, it exhibits superior electron transfer and delocalization ability, thereby enhancing its photocatalytic performance. The Optimal photocatalyst can degrade 98.78 % of 20 mg/L tetracycline (TC) within 15 min. Additionally, we introduced a novel proof strategy for investigating the photoelectron transfer path, creatively demonstrating the synergistic dual channels effect, which can be attributed to the carbonyl density and light-excitation degree. Notably, even under low-power light sources, it achieved complete inactivation of Escherichia coli within just 7 mins, far surpassing current cutting-edge research. This theoretical framework holds promise for broader applications within related studies.

General Stacking Theory for Altermagnetism in Bilayer Systems

Two-dimensional (2D) altermagnetism was recently proposed to be attainable in twisted antiferromagnetic bilayers providing an experimentally feasible approach to realize it in 2D materials. Nevertheless, a comprehensive understanding of the mechanism governing the appearance of altermagnetism in bilayer systems is still absent. In the present Letter, we address this gap by introducing a general stacking theory (GST) as a key condition for the emergence of altermagnetism in bilayer systems. The GST provides straightforward criteria to predict whether a bilayer demonstrates altermagnetic spin splitting, solely based on the layer groups of the composing monolayers. According to the GST, only seven point groups of bilayers facilitate the emergence of altermagnetism. It is revealed that, beyond the previously proposed antiferromagnetic twisted Van der Waals stacking, altermagnetism can even emerge in bilayers formed through the symmetrically restricted direct stacking of two monolayers. By combining the GST and first-principles calculations, we present illustrative examples of bilayers demonstrating altermagnetism. Our work establishes a robust framework for designing diverse bilayer systems with altermagnetism, thereby opening up new avenues for both fundamental research and practical applications in this field.

Imaging Strain-Controlled Magnetic Reversal in Thin CrSBr

Two-dimensional materials are extraordinarily sensitive to external stimuli, making them ideal for studying fundamental properties and for engineering devices with new functionalities. One such stimulus, strain, affects the magnetic properties of the layered magnetic semiconductor CrSBr to such a degree that it can induce a reversible antiferromagnetic-to-ferromagnetic phase transition. Using scanning SQUID-on-lever microscopy, we directly image the effects of spatially inhomogeneous strain on the magnetization of layered CrSBr, as it is polarized by a field applied along its easy axis. The evolution of this magnetization and the formation of domains is reproduced by a micromagnetic model, which incorporates the spatially varying strain and the corresponding changes in the local interlayer exchange stiffness. The observed sensitivity to small strain gradients along with similar images of a nominally unstrained CrSBr sample suggest that unintentional strain inhomogeneity influences the magnetic behavior of exfoliated samples.

Constructing the Fulde-Ferrell-Larkin-Ovchinnikov State in a CrOCl/NbSe2 van der Waals Heterostructure

Time reversal symmetry breaking in superconductors, resulting from external magnetic fields or spontaneous magnetization, often leads to unconventional superconducting properties. In this way, an intrinsic phenomenon called the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state may be realized by the Zeeman effect. Here, we construct the FFLO state in an artificial CrOCl/NbSe2 van der Waals (vdW) heterostructure by utilizing the superconducting proximity effect of NbSe2 flakes. The proximity-induced superconductivity demonstrates a considerably weak gap of about 0.12 meV, and the in-plane upper critical field reveals the behavior of the FFLO state. First-principles calculations uncover the origin of the proximitized superconductivity, which indicates the importance of Cr vacancies or line defects in CrOCl. Moreover, the FFLO state could be induced by the inherent large spin splitting in CrOCl. Our findings not only provide a practical scheme for constructing the FFLO state but also inspire the discovery of an exotic FFLO state in other two-dimensional vdW heterostructures.

FePd2Te2: An Anisotropic Two-Dimensional Ferromagnet with One-Dimensional Fe Chains

Two-dimensional (2D) magnets have attracted significant attention in recent years due to their importance in the research on both fundamental physics and spintronic applications. Here, we report the discovery of a new ternary compound FePd2Te2. It features a layered quasi-2D crystal structure with 1D Fe zigzag chains extending along the b-axis in the cleavage plane. Single crystals of FePd2Te2 with centimeter size could be grown. Density functional theory calculations, mechanical exfoliation, and atomic force microscopy on these crystals reveal that they are 2D materials that can be thinned down to ∼5 nm. Magnetic characterization shows that FePd2Te2 is an easy-plane ferromagnet with TC ∼ 183 K and strong in-plane uniaxial magnetic anisotropy. Magnetoresistance and the anomalous Hall effect demonstrate that ferromagnetism could be maintained in FePd2Te2 flakes with large coercivity. A crystal twinning effect is observed by scanning tunneling microscopy which makes the Fe chains right angle bent in the cleavage plane and creates an intriguing spin texture. Besides, a large electronic specific heat coefficient of up to γ ∼ 32.4 mJ mol-1 K-2 suggests FePd2Te2 is a strongly correlated metal. Our results show that FePd2Te2 is a correlated anisotropic 2D magnet that may attract multidisciplinary research interests.

Anisotropic 2D van der Waals Magnets Hosting 1D Spin Chains

The exploration of 1D magnetism, frequently portrayed as spin chains, constitutes an actively pursued research field that illuminates fundamental principles in many-body problems and applications in magnonics and spintronics. The inherent reduction in dimensionality often leads to robust spin fluctuations, impacting magnetic ordering and resulting in novel magnetic phenomena. Here, structural, magnetic, and optical properties of highly anisotropic 2D van der Waals antiferromagnets that uniquely host spin chains are explored. First-principle calculations reveal that the weakest interaction is interchain, leading to essentially 1D magnetic behavior in each layer. With the additional degree of freedom arising from its anisotropic structure, the structure is engineered by alloying, varying the 1D spin chain lengths using electron beam irradiation, or twisting for localized patterning, and spin textures are calculated, predicting robust stability of the antiferromagnetic ordering. Comparing with other spin chain magnets, these materials are anticipated to bring fresh perspectives on harvesting low-dimensional magnetism.

Electrochemical Intercalation and Exfoliation of CrSBr into Ferromagnetic Fibers and Nanoribbons

Recent studies dedicated to layered van der Waals crystals have attracted significant attention to magnetic atomically thin crystals offering unprecedented opportunities for applications in innovative magnetoelectric, magneto-optic, and spintronic devices. The active search for original platforms for the low-dimensional magnetism study has emphasized the entirely new magnetic properties of two dimensional (2D) semiconductor CrSBr. Herein, for the first time, the electrochemical exfoliation of bulk CrSBr in a non-aqueous environment is demonstrated. Notably, crystal cleavage governed by the structural anisotropy occurred along two directions forming atomically thin and few-layered nanoribbons. The exfoliated material possesses an orthorhombic crystalline structure and strong optical anisotropy, showing the polarization dependencies of Raman signals. The antiferromagnetism exhibited by multilayered CrSBr gives precedence to ferromagnetic ordering in the revealed CrSBr nanostructures. Furthermore, the potential application of CrSBr nanoribbons is pioneered for electrochemical photodetector fabrication and demonstrates its responsivity up to 30 µA cm-2 in the visible spectrum. Moreover, the CrSBr-based anode for lithium-ion batteries exhibited high performance and self-improving abilities. This anticipates that the results will pave the way toward the future study of CrSBr and practical applications in magneto- and optoelectronics.

Ultrafast Exciton Dynamics in the Atomically Thin van der Waals Magnet CrSBr

Among atomically thin semiconductors, CrSBr stands out as both its bulk and monolayer forms host tightly bound, quasi-one-dimensional excitons in a magnetic environment. Despite its pivotal importance for solid-state research, the exciton lifetime has remained unknown. While terahertz polarization probing can directly trace all excitons, independently of interband selection rules, the corresponding large far-field foci substantially exceed the lateral sample dimensions. Here, we combine terahertz polarization spectroscopy with near-field microscopy to reveal a femtosecond decay of paramagnetic excitons in a monolayer of CrSBr, which is 30 times shorter than the bulk lifetime. We unveil low-energy fingerprints of bound and unbound electron-hole pairs in bulk CrSBr and extract the nonequilibrium dielectric function of the monolayer in a model-free manner. Our results demonstrate the first direct access to the ultrafast dielectric response of quasi-one-dimensional excitons in CrSBr, potentially advancing the development of quantum devices based on ultrathin van der Waals magnets.

Polarization-Resolved Position-Sensitive Self-Powered Binary Photodetection in Multilayer Janus CrSBr

Recent progress in polarization-resolved photodetection based on low-symmetry 2D materials has formed the basis of cutting-edge optoelectronic devices, including quantum optical communication, 3D image processing, and sensing applications. Here, we report an optical polarization-resolving photodetector (PD) fabricated from multilayer semiconducting CrSBr single crystals with high structural anisotropy. We have demonstrated self-powered photodetection due to the formation of Schottky junctions at the Au-CrSBr interfaces, which also caused the photocurrent to display a position-sensitive and binary nature. The self-biased CrSBr PD showed a photoresponsivity of ∼0.26 mA/W with a detectivity of 3.4 × 108 Jones at 514 nm excitation of fluency (0.42 mW/cm2) under ambient conditions. The optical polarization-induced photoresponse exhibits a large dichroic ratio of 3.4, while the polarization is set along the a- and the b-axes of single-crystalline CrSBr. The PD also showed excellent stability, retaining >95% of the initial photoresponsivity in ambient conditions for more than five months without encapsulation. Thus, we demonstrate CrSBr as a fascinating material for ultralow-powered optical polarization-resolving optoelectronic devices for cutting-edge technology.

Lightning the Spin: Harnessing the Potential of 2D Magnets in Opto-Spintronics

Since the emergence of 2D magnets in 2017, the diversity of these materials has greatly expanded. Their 2D nature (atomic-scale thickness) endows these magnets with strong magnetic anisotropy, layer-dependent and switchable magnetic order, and quantum-confined quasiparticles, which distinguish them from conventional 3D magnetic materials. Moreover, the 2D geometry facilitates light incidence for opto-spintronic applications and potential on-chip integration. In analogy to optoelectronics based on optical-electronic interactions, opto-spintronics use light-spin interactions to process spin information stored in the solid state. In this review, opto-spintronics is divided into three types with respect to the wavelengths of radiation interacting with 2D magnets: 1) GHz (microwave) to THz (mid-infrared), 2) visible, and 3) UV to X-rays. It is focused on the recent research advancements on the newly discovered mechanisms of light-spin interactions in 2D magnets and introduces the potential design of novel opto-spintronic applications based on these interactions.

Ferromagnetic Interlayer Coupling in CrSBr Crystals Irradiated by Ions

Layered magnetic materials are becoming a major platform for future spin-based applications. Particularly, the air-stable van der Waals compound CrSBr is attracting considerable interest due to its prominent magneto-transport and magneto-optical properties. In this work, we observe a transition from antiferromagnetic to ferromagnetic behavior in CrSBr crystals exposed to high-energy, non-magnetic ions. Already at moderate fluences, ion irradiation induces a remanent magnetization with hysteresis adapting to the easy-axis anisotropy of the pristine magnetic order up to a critical temperature of 110 K. Structure analysis of the irradiated crystals in conjunction with density functional theory calculations suggests that the displacement of constituent atoms due to collisions with ions and the formation of interstitials favors ferromagnetic order between the layers.

Recent advances in 2D van der Waals magnets: Detection, modulation, and applications

The emergence of two-dimensional (2D) van der Waals magnets provides an exciting platform for exploring magnetism in the monolayer limit. Exotic quantum phenomena and significant potential for spintronic applications are demonstrated in 2D magnetic crystals and heterostructures, which offer unprecedented possibilities in advanced formation technology with low power and high efficiency. In this review, we summarize recent advances in 2D van der Waals magnetic crystals. We focus mainly on van der Waals materials of truly 2D nature with intrinsic magnetism. The detection methods of 2D magnetic materials are first introduced in detail. Subsequently, the effective strategies to modulate the magnetic behavior of 2D magnets (e.g., Curie temperature, magnetic anisotropy, magnetic exchange interaction) are presented. Then, we list the applications of 2D magnets in the spintronic devices. We also highlight current challenges and broad space for the development of 2D magnets in further studies.

Origin and regulation of triaxial magnetic anisotropy in the ferromagnetic semiconductor CrSBr monolayer

Magnetic anisotropy plays a vital role in stabilizing the long-range magnetic order of two-dimensional ferromagnetic systems. In this work, using the first-principles method, we systematically explored the triaxial magnetic anisotropic properties of a ferromagnetic semiconductor CrSBr monolayer, which is recently exfoliated from its bulk. Further analysis shows that the triaxial magnetic anisotropic properties originate from the coexistence of the magnetic dipole-dipole interaction (shape anisotropy) and the spin-orbit coupling interaction (magnetocrystalline anisotropy). Interestingly, the shape anisotropy, which has been neglected in most previous works, dominates over the magnetocrystalline anisotropy. Besides, the experimental Curie temperature of the CrSBr monolayer is well reproduced using Monte Carlo simulations. What is more, the easy magnetic axes and ferromagnetism in the CrSBr monolayer can be manipulated by strains and are relatively more susceptible to the uniaxial strain in the x direction. Our study not only explains the mechanism of triaxial magnetic anisotropy of the CrSBr monolayer, but also sheds light on how to tune the magnetic anisotropy and Curie temperature in ferromagnetic monolayers.

Interplay between Optical Emission and Magnetism in the van der Waals Magnetic Semiconductor CrSBr in the Two-Dimensional Limit

The van der Waals semiconductor metamagnet CrSBr offers an ideal platform for studying the interplay between optical and magnetic properties in the two-dimensional limit. Here, we carried out an exhaustive optical characterization of this material by means of temperature- and magnetic-field-dependent photoluminescence (PL) on flakes of different thicknesses down to the monolayer. We found a characteristic emission peak that is quenched upon switching the ferromagnetic layers from an antiparallel to a parallel configuration and exhibits a temperature dependence different from that of the peaks commonly ascribed to excitons. The contribution of this peak to the PL is boosted around 30-40 K, coinciding with the hidden order magnetic transition temperature. Our findings reveal the connection between the optical and magnetic properties via the ionization of magnetic donor vacancies. This behavior enables a useful tool for the optical reading of the magnetic states in atomically thin layers of CrSBr and shows the potential of the design of 2D heterostructures with magnetic and excitonic properties.

The Bulk van der Waals Layered Magnet CrSBr is a Quasi-1D Material

Correlated quantum phenomena in one-dimensional (1D) systems that exhibit competing electronic and magnetic order are of strong interest for the study of fundamental interactions and excitations, such as Tomonaga-Luttinger liquids and topological orders and defects with properties completely different from the quasiparticles expected in their higher-dimensional counterparts. However, clean 1D electronic systems are difficult to realize experimentally, particularly for magnetically ordered systems. Here, we show that the van der Waals layered magnetic semiconductor CrSBr behaves like a quasi-1D material embedded in a magnetically ordered environment. The strong 1D electronic character originates from the Cr-S chains and the combination of weak interlayer hybridization and anisotropy in effective mass and dielectric screening, with an effective electron mass ratio of m_X^e/m_Y^e ∼ 50. This extreme anisotropy experimentally manifests in strong electron-phonon and exciton-phonon interactions, a Peierls-like structural instability, and a Fano resonance from a van Hove singularity of similar strength to that of metallic carbon nanotubes. Moreover, because of the reduced dimensionality and interlayer coupling, CrSBr hosts spectrally narrow (1 meV) excitons of high binding energy and oscillator strength that inherit the 1D character. Overall, CrSBr is best understood as a stack of weakly hybridized monolayers and appears to be an experimentally attractive candidate for the study of exotic exciton and 1D-correlated many-body physics in the presence of magnetic order.

Sensing the Local Magnetic Environment through Optically Active Defects in a Layered Magnetic Semiconductor

Atomic-level defects in van der Waals (vdW) materials are essential building blocks for quantum technologies and quantum sensing applications. The layered magnetic semiconductor CrSBr is an outstanding candidate for exploring optically active defects because of a direct gap, in addition to a rich magnetic phase diagram, including a recently hypothesized defect-induced magnetic order at low temperature. Here, we show optically active defects in CrSBr that are probes of the local magnetic environment. We observe a spectrally narrow (1 meV) defect emission in CrSBr that is correlated with both the bulk magnetic order and an additional low-temperature, defect-induced magnetic order. We elucidate the origin of this magnetic order in the context of local and nonlocal exchange coupling effects. Our work establishes vdW magnets like CrSBr as an exceptional platform to optically study defects that are correlated with the magnetic lattice. We anticipate that controlled defect creation allows for tailor-made complex magnetic textures and phases with direct optical access.