Distinguishing arc-melted from fire-melted beads on copper wires is important for interpreting electrical causation in fire investigations. Magnetization measurements were applied as a non-destructive method for characterizing the magnetic behavior of the two bead types at room temperature. Arc-melted beads exhibited paramagnetic behavior, whereas fire-melted beads showed predominantly diamagnetic behavior. This contrast is interpreted in relation to the oxide phases formed under different oxygen environments, where CuO is favored in oxygen-rich conditions and Cu2O becomes relatively more favorable in oxygen-deficient conditions. The absence of hysteresis with no ferromagnetic contaminants in the energy-dispersive spectroscopy results confirms that the paramagnetic signal in arc-melted beads is intrinsic to the CuO-containing oxide. Arc-melted beads also showed weak magnetic anisotropy, whereas fire-melted beads showed isotropic behavior. These results suggest that non-destructive magnetization measurements may serve as a useful preliminary screening method for bead discrimination, preserving the specimen for subsequent analysis.

Van der Waals moiré superlattices can host interfacial ferroelectric domains, whose arrangement is influenced by the topology of nodes. Here, we investigate the dynamic behavior and topological protection of node types in twisted WSe2 homobilayers under local mechanical perturbations. Dark-field transmission electron microscopy and Kelvin probe force microscopy reveal symmetric sixfold nodes as well as non-ideal nodes, including fourfold and eightfold types. Mechanical loading with an atomic force microscopy tip shows striking contrasts in node motion: symmetric sixfold nodes are dynamically robust, whereas an asymmetric sixfold node can be displaced and guided by the tip scan direction, as confirmed by quasi-static loading simulations. Furthermore, we observe a fourfold node that fractured due to its lack of topological protection. These findings establish a direct link between node topology and mechanical response, providing fundamental insights into moiré ferroelectricity and advancing our understanding of topological defects in two-dimensional van der Waals materials.

Optical spectroscopic tools are extensively utilized in the study of van der Waals magnetic materials. The small volume of atomically thin specimens renders traditional measurement tools such as magnetic susceptibility measurements or neutron scattering experiment inadequate. Optical measurements, on the other hand, can have a spatial resolution on the order of the focus size of the probing beam and are often sensitive enough to probe atomically thin samples. Several optical spectroscopy techniques have demonstrated to be capable of probing various physical properties of these materials. Raman spectroscopy, terahertz spectroscopy, photoluminescence, optical absorption, and second harmonic generation have been applied to various magnetic van der Waals materials and have shown to be particularly powerful in the studies of antiferromagnetic van der Waals materials.

The recently discovered magnetic exciton in the van der Waals (vdW) antiferromagnet NiPS3 exemplifies these phenomena, exhibiting several distinctive characteristics. Despite extensive investigation, much of its physics remains unresolved, with key questions about why the NiPS3 magnetic exciton is so sharp and optically bright despite the nominally spin-forbidden transition, posing significant challenges to a proper understanding and practical manipulation of the exciton. An urgent question is to what extent it is due to chemical disorder, magnetic weakening, lattice modification, or intrinsic instability of the bright exciton itself: answers to which will put stringent constraints on possible theoretical models. Here we address these questions using hydrostatic pressure as a clean, continuous, reversible, and in-situ tuning parameter. We find that the sharp photoluminescence peak is drastically suppressed by as little as 0.4 GPa and completely quenched by 1.5 GPa, with demonstrating its reversibility. Crucially, this bright-to-dark conversion occurs without magnetic, crystallographic, or electronic reconstruction despite an increase in the Neel temperature, as established by Raman, X-ray absorption, nuclear magnetic resonance spectroscopy, and first-principles many-body calculations. Our results demonstrate that the optical brightness of the magnetic exciton is independent of chemical disorder, lattice expansion, and weakening of magnetic order, indicating that a higher-order correlated mechanism governs the bright exciton. We further propose experimentally constrained microscopic scenarios involving exciton pairing, crystal-field-controlled spin-orbit mixing, and symmetry breaking, providing a framework for future tests of entangled magnetic exciton in correlated quantum magnets.

Twisted transition metal dichalcogenide (TMD) bilayers have garnered significant attention due to the emergence of unconventional quantum phenomena, such as sliding ferroelectricity in multidomain TMD bilayers with domain walls (DWs). Thus, understanding their atomic reconstruction is essential for elucidating the origin of such properties. While atomic reconstruction has been observed in twisted bilayers with small lattice mismatch, large-mismatch systems have generally been thought to retain incommensurate moiré superlattices as the substantial bond deformation required for reconstruction renders it energetically unfavorable. Here, we demonstrate that MoSe2/MoS2 heterobilayers with a large lattice mismatch can reconstruct into commensurate domain structures by encapsulation annealing at a high temperature of >1100 °C. This process supplied sufficient thermal energy and vertical compression to enable local atomic rearrangements and facilitates vacancy-assisted chalcogen exchange across the interface, where Se and S atoms migrate between the two layers through chalcogen vacancies. This interlayer atomic diffusion results in the formation of the MoSe2-xSx/MoS2-xSex alloy, effectively reducing the lattice mismatch and enabling the formation of commensurate AB/BA domains separated by tensile saddle point (SP) boundaries. These tensile boundaries reflect residual in-plane strain from the reduced lattice mismatch, distinct from the shear-type boundaries observed in lattice-matched systems. Spectroscopic analysis further reveals enhanced interlayer coupling and room temperature valley polarization in the alloyed heterobilayer. Our findings establish a conceptual framework for structural reconstruction in large-lattice-mismatched van der Waals (vdW) heterostructures, where chalcogen alloying enables domain formation previously thought inaccessible, thereby offering additional pathways to engineer the structural and optoelectronic properties of 2D materials.

Two-dimensional (2D) semiconductors, particularly transition metal dichalcogenides (TMDs), are promising for advanced electronics beyond silicon1-3. Traditionally, TMDs are epitaxially grown on crystalline substrates by chemical vapour deposition. However, this approach requires post-growth transfer to target substrates, which makes controlling thickness and scalability difficult. Here we introduce a method called hypotaxy ('hypo' meaning downward and 'taxy' meaning arrangement), which enables wafer-scale single-crystal TMD growth directly on various substrates, including amorphous and lattice-mismatched substrates, while preserving crystalline alignment with an overlying 2D template. By sulfurizing or selenizing a pre-deposited metal film under graphene, aligned TMD nuclei form, coalescing into a single-crystal film as graphene is removed. This method achieves precise MoS2 thickness control from monolayer to hundreds of layers on diverse substrates, producing 4-inch single-crystal MoS2 with high thermal conductivity (about 120 W m-1 K-1) and mobility (around 87 cm2 V-1 s-1). Furthermore, nanopores created in graphene using oxygen plasma treatment allow MoS2 growth at a lower temperature of 400 °C, compatible with back-end-of-line processes. This hypotaxy approach extends to other TMDs, such as MoSe2, WS2 and WSe2, offering a solution to substrate limitations in conventional epitaxy and enabling wafer-scale TMDs for monolithic three-dimensional integration.

Transition metal dichalcogenides (TMDs), including MoS2 and WS2, exhibit distinct optical and electrical properties that are highly dependent on their thickness and stacking order. However, controlling these parameters during synthesis remains a challenge. Here, we present a synthesis technique for multilayer single-crystal tungsten disulfide (WS2) utilizing a liquid-phase tungsten oxide (WOx)-driven physical vapor deposition (PVD) method. Our approach successfully addresses the challenges of manipulating the stacking order in TMD multilayers, which is vital for harnessing their unique properties for advanced technological applications. By employing a molten WOx intermediate, we achieved controlled growth of WS2 single crystals with varied layer numbers and stacking configurations of 2H and 3R. This method not only facilitates the fabrication of TMD layers beyond monolayers but also provides the ability to tailor their stacking orders. Our findings offer a pathway for the growth of multilayer TMDs, underscoring the potential for scalable production of high-quality single-crystal TMDs for industrial use.

Previous high-pressure studies about WTe2 have reported divergent critical pressures for structural and electronic phase transitions, obscuring a comprehensive understanding about the complex quantum phases. In this work, we precisely assign the structural phase evolutions and relevant electronic changes of type-II Weyl semimetals, WTe2 and MoTe2, using various optical methods. We confirm that the Td to 1T′ structural phase transition occurs at about 2.5 GPa concomitantly with the previously reported decrease of magnetoresistance and emergence of superconductivity. Notably, electron-phonon coupling remains intact despite pressure-dependent variations in structural symmetry and atomic bond strengths, providing crucial insights into the origin of superconductivity in transition metal ditellurides. We also demonstrate an additional structural transition at about 10 GPa, possibly to a triclinic 1T″ structure, which has a significant influence on the electronic structure due to intra-layer distortion of atomic positions. Our findings on distinct evolutions of inter-layer and intra-layer structural parameters offer a generic understanding of the relationship between anisotropic bond strength in the van der Waals materials and pressure-dependent structural changes. We assign the high pressure structural phase of transition metal ditelluride (MTe2, M = W or Mo), one of the most extensively investigated material groups. From the accurate assignment of structural phases, we offer comprehensive understanding of the electronic and topological phase evolutions. We can also clearly demonstrate anisotropic pressure-dependent structural evolutions for transition metal ditellurides, which can be generic behaviors of two-dimensional van der Waals materials under pressure.

Magnetism has played a central role in the long and rich history of modern condensed matter physics, with many foundational insights originating from theoretical studies of two-dimensional (2D) spin systems. The discovery of 2D van der Waals (vdW) magnets has revolutionized this area by providing real, atomically thin magnetic systems for experimental investigation. Since the first experimental reports of antiferromagnetic vdW insulators in 2016 - followed by studies on ferromagnetic vdW systems in 2017 - the field has witnessed rapid and expansive growth, with more than two dozen vdW magnetic materials now identified, including both ferro- and antiferromagnets. In this review, we present a comprehensive overview of the major scientific and technological developments in this rapidly evolving field. These include experimental realizations of various 2D spin Hamiltonians as well as unexpected phenomena such as magnetic excitons, Floquet-engineered states, and light-induced metastable magnetic phases. In parallel, 2D vdW magnets have shown significant promise in spintronics and related applications, offering a new platform for engineering quantum functionalities. We organize this review by tracing the historical development of the field, synthesizing key milestones, and highlighting its broader impact across condensed matter physics and materials science. We conclude with an Outlook section that outlines several promising directions for future research, aiming to chart a path forward in this vibrant and still rapidly growing area.

Interlayer interactions in few-layer NiPS3 were investigated by analyzing low-frequency interlayer vibration modes and Davydov splitting of an intralayer, A1g vibration mode at ~255 cm–1 by Raman spectroscopy as a function of temperature. The interlayer force constants were estimated from the low-frequency Raman spectra by using the linear chain model. The out-of-plane direction interlayer force constant could also be estimated separately from the Davydov splitting, which agrees well with the linear chain model analysis. The dependence of the low-frequency shear and breathing modes and the Davydov splitting on the number of layers provide a unique, reliable tool for determining the number of layers.

Twisted transition metal dichalcogenide (TMD) bilayers have garnered significant attention due to the emergence of unconventional quantum phenomena, such as sliding ferroelectricity in multidomain TMD bilayers with domain walls (DWs). Thus, understanding their atomic reconstruction is essential for elucidating the origin of such properties. While atomic reconstruction has been observed in twisted bilayers with small lattice mismatch, large-mismatch systems have generally been thought to retain incommensurate moiré superlattices as the substantial bond deformation required for reconstruction renders it energetically unfavorable. Here, we demonstrate that MoSe2/MoS2 heterobilayers with a large lattice mismatch can reconstruct into commensurate domain structures by encapsulation annealing at a high temperature of >1100 °C. This process supplied sufficient thermal energy and vertical compression to enable local atomic rearrangements and facilitates vacancy-assisted chalcogen exchange across the interface, where Se and S atoms migrate between the two layers through chalcogen vacancies. This interlayer atomic diffusion results in the formation of the MoSe2-xSx/MoS2-xSex alloy, effectively reducing the lattice mismatch and enabling the formation of commensurate AB/BA domains separated by tensile saddle point (SP) boundaries. These tensile boundaries reflect residual in-plane strain from the reduced lattice mismatch, distinct from the shear-type boundaries observed in lattice-matched systems. Spectroscopic analysis further reveals enhanced interlayer coupling and room temperature valley polarization in the alloyed heterobilayer. Our findings establish a conceptual framework for structural reconstruction in large-lattice-mismatched van der Waals (vdW) heterostructures, where chalcogen alloying enables domain formation previously thought inaccessible, thereby offering additional pathways to engineer the structural and optoelectronic properties of 2D materials.

Studying antiferromagnetic domains is essential for fundamental physics and potential spintronics applications. Despite their importance, few systematic studies have been performed on antiferromagnet (AFM) domains with high spatial resolution in van der Waals (vdW) materials, and direct probing of the Néel vectors remains challenging. In this work, we found multidomain states in the vdW AFM NiPS3, a material extensively investigated for its unique magnetic exciton. We employed photoemission electron microscopy combined with the X-ray magnetic linear dichroism (XMLD-PEEM) to image the NiPS3's magnetic structure. The nanometer-spatial resolution of XMLD-PEEM allows us to determine local Néel vector orientations and discover thermally fluctuating Néel vectors that are independent of the crystal symmetry even at 65 K, well below the TN of 155 K. We demonstrate that an in-plane orbital moment of the Ni ion is responsible for the weak magnetocrystalline anisotropy. The observed thermal fluctuations of the antiferromagnetic domains may explain the broadening of magnetic exciton peaks at higher temperatures.

The temperature dependence of the Raman spectrum of exfoliated CrPS4 samples was investigated by polarized Raman spectroscopy with three excitation sources between 4 K and room temperature (293 K). The peak positions and the polarization dependences of the 14 observed Raman modes are consistent with the previous report at room temperature. However, there are some critical changes in the Raman spectrum at low temperatures. For the bulk sample, the mode at similar to 306 cm(-1) is polarized along the b-axis of the crystal at room temperature but is slightly polarized along the a-axis at low temperatures. In addition, a mode at similar to 188 cm(-1) emerges, and its intensity increases dramatically below the N & eacute;el temperature. Both of these changes were most prominent with a 2.41 eV excitation source. Similar changes were observed in few-layer samples down to 2-layers.

The low-frequency interlayer vibration modes in bilayer-MoS2/monolayer-WSe2 heterostructures were investigated to study the modification of interlayer interactions due to the moire periodicity. The interplay of the interlayer interaction within bilayer MoS2 and the interfacial interaction between the two materials results in rich features in the phonon spectra. Several shear and breathing modes are observed for samples with small twist angles (<10(degrees)), whereas only one shear and two breathing modes are observed for larger twist angles. For larger twist angles, the interfacial interaction between the two materials amounts to similar to 75% of the intrinsic interlayer interaction between the MoS2 layers. The phonon spectrum evolves non-monotonically as the twist angle increases, which is explained with the help of atomistic calculations.

We investigated interlayer modes of few-layer HfX2 (X = S, Se) by using low-frequency micro-Raman spectroscopy with three excitation energies (1.96 eV, 2.33 eV, 2.54 eV) under vacuum condition (similar to 10(-6)Torr). We observed interlayer modes in HfSe2 when the 2.54 eV excitation energy was used. The low-frequency Raman spectra reveal a series of shear and breathing modes (<50 cm(-1)) that are helpful for identifying the number of layers. The in-plane E-g and out-of-plane A(1g) modes of HfSe2 are located at similar to 150 cm(-1) and similar to 200 cm(-1), respectively. In HfS2, in-plane E-g and out-of-plane A(1g) optical phonons are observed at similar to 260 cm(-1) and similar to 337 cm(-1), respectively. The in-plane and out-of-plane force constants of atomically thin HfSe2 are obtained to be 1.87 x 10(19)N m(-3) and 6.55 x 10(19)N m(-3), respectively, by fitting the observed interlayer modes using the linear chain model. These results provide valuable information on materials parameters for device designs using atomically-thin layered HfX2 (X = S, Se).

Abstractγ-GeSe is a newly identified polymorph among group-IV monochalcogenides, characterized by a distinctive interatomic bonding configuration. Despite its promising applications in electrical and thermal domains, the experimental verification of its mechanical and thermal properties remains unreported. Here, we experimentally characterize the in-plane Young’s modulus (E) and thermal conductivity ($$\:\kappa\:$$) of γ-GeSe. The mechanical vibrational modes of freestanding γ-GeSe flakes are measured using optical interferometry. Nano-indentation via atomic force microscopy is also conducted to induce mechanical deformation and to extract the E. Comparison with finite-element simulations reveals that the E is 97.3$$\:\pm\:$$7.5 GPa as determined by optical interferometry and 109.4$$\:\pm\:$$13.5 GPa as established through the nano-indentation method. Additionally, optothermal Raman spectroscopy reveals that γ-GeSe has a lattice thermal conductivity of 2.3 $$\:\pm\:$$ 0.4 Wm−1K−1 and a total thermal conductivity of 7.5 $$\:\pm\:$$ 0.4 Wm−1K−1 in the in-plane direction at room temperature. The notably high $$\:E/\kappa\:$$ ratio in γ-GeSe, compared to other layered materials, underscores its distinctive structural and dynamic characteristics.

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.

Monolayer transition metal dichalcogenides (TMDs) have emerged as highly promising candidates for optoelectronic applications due to their direct band gap and strong light-matter interactions. However, exfoliated TMDs have demonstrated optical characteristics that fall short of expectations, primarily because of significant defects and associated doping in the synthesized TMD crystals. Here, we report the improvement of optical properties in monolayer TMDs of MoS2, MoSe2, WS2, and WSe2, by hBN-encapsulation annealing. Monolayer WSe2 showed 2000% enhanced photoluminescence quantum yield (PLQY) and 1000% increased lifetime after encapsulation annealing at 1000 °C, which are attributed to dominant radiative recombination of excitons through dedoping of monolayer TMDs. Furthermore, after encapsulation annealing, the transport characteristics of monolayer WS2 changed from n-type to ambipolar, along with an enhanced hole transport, which also support dedoping of annealed TMDs. This work provides an innovative approach to elevate the optical grade of monolayer TMDs, enabling the fabrication of high-performance optoelectronic devices.

Elemental phosphorus exhibits fascinating structural varieties and versatile properties. The unique nature of phosphorus bonds can lead to the formation of extremely complex structures, and detailed structural information on some phosphorus polymorphs is yet to be investigated. In this study, we investigated an unidentified crystalline phase of phosphorus, type-II red phosphorus (RP), by combining state-of-the-art structural characterization techniques. Electron diffraction tomography, atomic-resolution scanning transmission electron microscopy (STEM), powder X-ray diffraction, and Raman spectroscopy were concurrently used to elucidate the hidden structural motifs and their packing in type-II RP. Electron diffraction tomography, performed using individual crystalline nanowires, was used to identify a triclinic unit cell with volume of 5330 Å3 , which is the largest unit cell for elemental phosphorus crystals up to now and contains approximately 250 phosphorus atoms. Atomic-resolution STEM imaging, which was performed along different crystal-zone axes, confirmed that the twisted wavy tubular motif is the basic building block of type-II RP. Our study discovered and presented a new variation of building blocks in phosphorus, and it provides insights to clarify the complexities observed in phosphorus as well as other relevant systems.

The unique discovery of the magnetic exciton in van der Waals antiferromagnet NiPS3 arises 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-xCdxPS3 using 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.