We report the emergence of a two-dimensional (2D) polar metal phase in van der Waals compound FePSe_3 under moderate pressures. This layered material is a Mott insulator with antiferromagnetic order under ambient conditions. We show that FePSe_3 uniquely allows tuning a 2D correlated insulator into an exotic metal state where a loss of inversion symmetry leads to periodic polar displacements of ions, within a conducting phase - a polar metal. Our combined synchrotron and neutron diffraction data allow us to present a long-sought, unambiguous high-pressure structural model and show the polar displacements of this new phase. We also observe the suppression of magnetic ordering at the insulator-to-metal transition correspondent with this structural change. Our work outlines a comprehensive temperature-pressure phase diagram of FePSe_3, combining detailed structural, magnetic and transport data. The high-pressure phase exhibits activated semiconductor behavior at high temperatures, a T^2-dependence in its resistivity at lower temperatures - despite the conditions required for a `good metal' Fermi-Liquid description not being met in this case - and a low-temperature resistivity upturn which is suppressed as the system is tuned away from the concomitant transitions. The realisation of a tunable 2D polar metal state in FePSe_3 due to the loss of its inversion symmetry combined with pressure-induced metallicity offers a promising new platform to investigate this exotic phase at accessible pressures.

Strong quantum fluctuations and unconventional spin dynamics are well established in the spin-1/2 triangular lattice Heisenberg antiferromagnet. However, their survival in the spin-1 case remains an open question. We investigate the spin dynamics of K_2Ni(SeO_3)_2, a nearly ideal spin-1 triangular lattice Heisenberg antiferromagnet, using inelastic neutron scattering. Below the ordering temperature T_ N, we observe coherent one-magnon excitations coexisting with a broad high-energy continuum. Two complementary approaches, a spectrally consistent 1/S-corrected spin wave theory and a beyond-mean-field Schwinger boson theory, reproduce different facets of the continuum. Neither alone is complete, demonstrating substantial quantum fluctuations survive for S=1 and are reflected primarily in the spectral distribution of the continuum. Above T_ N, the continuum bandwidth is conserved while spectral weight is redistributed as magnons lose spatial coherence. Our results establish K_2Ni(SeO_3)_2 as a model triangular antiferromagnet, identifying bandwidth conservation and the distribution of spectral weight within the continuum as organizing principles to understand the spin dynamics of ordered quantum magnets beyond spin-1/2. Our results highlight the need for controlled calculations of the interacting multi-magnon sector of 2D antiferromagnets.

Topological spin textures are a spectacular manifestation of the chirality of the magnetic nanostructures protected by topology. Most known skyrmion systems are restricted to a topological charge of one, require an external magnetic field for stabilization, and are only reported in a few materials. Here, we investigate the possibility that the Kitaev anisotropic-exchange interaction stabilizes a higher-order skyrmion crystal in the insulating van der Waals magnet NiI2. We unveil and explain the incommensurate static and dynamic magnetic correlations across three temperature-driven magnetic phases of this compound using neutron scattering measurements, simulations, and modeling. Our parameter optimisation yields a minimal Kitaev-Heisenberg Hamiltonian for NiI2 which reproduces the experimentally observed magnetic excitations. Monte Carlo simulations for this model predict the emergence of the higher-order skyrmion crystal but neutron diffraction and optical experiments in the candidate intermediate temperature regime are inconclusive. We discuss possible deviations from the Kitaev-Heisenberg model that explains our results and conclude that NiI2, in addition to multiferroic properties in the bulk and few-layer limits, is a Kitaev bulk material proximate to the finite temperature higher-order skyrmion crystal phase.

CoxTaS2 (x approximate to 1/3) exhibits a spontaneous Hall effect from spin texture in antiferromagnets, with a tetrahedral triple-Q(3Q) order and uniform spin scalar chirality. Upon Co overdoping (x> 1/3), it undergoes a shift in magnetic ordering vectors from Q(m) = (1/2, 0, 0) to (1/3, 0, 0). Interestingly, the spontaneous Hall effect disappeared in the overdoped regime, which was originally attributed to the loss of 3Q order. However, a question remains whether a new type of 3Q order can exist with alternating chirality in the overdoped regime. To address this, we investigated Co0.336TaS2 using inelastic neutron scattering (INS), neutron diffraction, and optical dichroism, and found that INS data and spin-wave simulations support a 3Q order with alternating chirality. Moreover, neutron diffraction data show field-independent Bragg peaks, while linear dichroism detects no in-plane anisotropy, consistent with threefold rotation symmetry. Our data support the scenario of an alternating-chirality 3Q order in Co0.336TaS2, canceling the spontaneous Hall effect. This study highlights a combined neutron-optical approach to identify complex spin textures.

Quantum magnets with competing interactions often emerge from delicate balances among microscopic parameters, making it essential to disentangle intrinsic spin dynamics from extrinsic disorder effects. Here, we introduce a multimodal optical approach combining magneto-infrared spectroscopy with domain-resolved micro-Raman spectroscopy at high magnetic fields to reconstruct the intrinsic magnetic excitation spectrum of twinned crystals of the Kitaev-Heisenberg quantum magnet Na_3Co_2SbO_6. Far-infrared spectroscopy reveals multiple field-tunable magnetic excitations, but the intrinsic response is obscured by replica features arising from twin domains. By correlating magneto-infrared and domain-resolved Raman spectra, we isolate the single-domain magnon response and uncover a pronounced twofold in-plane magnon anisotropy. This anisotropy far exceeds that expected from the measured in-plane g-factor anisotropy and is instead dominated by anisotropic bond-dependent exchange interactions. By unifying high-field, high-resolution and spatially selective optical probes, our work establishes a broadly applicable framework for revealing intrinsic spin dynamics and constraining the spin Hamiltonian in multidomain quantum magnets.

α-RuCl_3 is a leading material for proximate Kitaev magnetism. We address the origin of the broad, Γ-point centered excitation continuum observed by inelastic neutron scattering at elevated temperatures in this compound. Using stochastic Landau-Lifshitz dynamics augmented with quantum-equivalent corrections, we reproduce the temperature-dependent dynamical spin structure factor across both the correlated and conventional paramagnetic regimes. A meta-analysis of 38 published exchange parameter sets identifies those most consistent with the full temperature evolution. A Bayesian optimization procedure is used to derive parameters that capture the low-energy star-like momentum dependence and the overall bandwidth of the continuum. Rescaling temperatures by the Curie–Weiss scale produces a collapse of spectral measures, demonstrating that the high-T dynamics are governed by correlated paramagnetism below θ_CW rather than by the Kitaev crossover to fractionalized excitations. Complementary 24-site exact diagonalization clarifies finite-size systematics at low temperature and the proximity to zigzag/incommensurate ordering. Beyond α-RuCl_3, our simulation pipeline provides a reproducible, data-driven framework to infer effective spin models in magnets that exhibit broad continua.

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.

Bond-dependent magnetic interactions, particularly those described by the Kitaev model, have emerged as a key pathway toward realizing unconventional magnetic states such as quantum spin liquids and topologically nontrivial excitations, including skyrmions. These interactions frustrate conventional magnetic order and give rise to rich collective behavior that continues to challenge both theory and experiment. While Kitaev physics has been extensively explored in the context of honeycomb magnets, direct evidence for its role in real materials remains scarce. Magnetic van der Waals (vdW) materials have emerged as a versatile platform for exploring low-dimensional electrical, magnetic, and correlated electronic phenomena, and provide a fertile ground for potential applications ranging from spintronics to multiferroic devices and quantum information technologies. Here, we demonstrate, through magneto-transmission, Faraday angle rotation, and magnetic circular dichroism measurements, that the magnetic excitation spectrum of NiI_2, a van der Waals multiferroic material, is more accurately captured by a Kitaev-based spin model than by the previously invoked helical spin framework.

We simulate the dynamical spin structure factor (DSSF) S(q,w) of the spin-1/2 Heisenberg antiferromagnetic chain using classical simulations. By employing Landau-Lifshitz Dynamics, we emulate quantum correlations through temperature-dependent corrections, including rescaling of magnetic dipoles and renormalization of exchange interactions. Our results demonstrate that the quantum-equivalent DSSF closely matches Quantum Monte-Carlo calculations for kBT/J 1, extending the applicability of classical dynamics to the challenging case of gapless excitations. At higher temperatures, our simulations comply with general predictions for uncorrelated paramagnetic fluctuations in the infinite temperature limit. Entanglement witnesses derived from the quantum-equivalent DSSF act as sensitive diagnostics for the quantum-to-classical crossover. Their reliability stems from their dependence on spectral features alone, enabling classical dynamics to emulate quantum thresholds without genuine entanglement. This framework also reproduces transverse spin correlations in finite magnetic fields, in agreement with quantum simulations. Together, our results establish quantum-corrected classical dynamics as a scalable and predictive tool for interpreting scattering experiments and exploring quantum correlations in strongly correlated spin systems.

The identification of quantum spin-liquid phases in Kitaev candidate materials remains a major experimental challenge. Since most Kitaev candidates develop antiferromagnetic (AFM) order at low temperatures, currently there is great interest on the field-induced magnetic disordered phase in these compounds that are distinct from (partially) polarized states. Recently, a cobaltate Na2Co2TeO6 has emerged as a promising Kitaev candidate with high-spin t52ge2g configuration and spin-orbit-entangled Jeff = 1/2 honeycomb lattice system. There are intensive studies on field-induced magnetic states and phase transitions under in-plane magnetic fields. In this study, we propose an intermediate disordered phase induced by an out-of-plane field along the c-axis, through high-field magnetization and magnetocaloric effect measurements. To explain the high-field behavior of Na2Co2TeO6, we develop an effective K-J-F-F' spin model featuring a dominant AFM Kitaev interaction. This framework uncovers an intermediate quantum spin-liquid phase, establishing the material as a unique platform for exploring Kitaev physics and field-induced quantum-disordered states.

Disorder significantly impacts the electronic properties of conducting quantum materials by inducing electron localization and thus altering the local density of states and electric transport. In insulating quantum magnetic materials, the effects of disorder are less understood and can drastically impact fluctuating spin states like quantum spin liquids. In the absence of transport tools, disorder is typically characterized using chemical methods or by semi-classical modeling of spin dynamics. This requires high magnetic fields that may not always be accessible. Here, we show that magnetization plateaus-incompressible states found in many quantum magnets-provide an exquisite platform to uncover small amounts of disorder, regardless of the origin of the plateau. Using optical magneto-spectroscopy on the Ising-Heisenberg triangular-lattice antiferromagnet K 2 Co ( SeO 3 ) 2 exhibiting a 1/3 magnetization plateau, we identify sharp spectroscopic lines, the fine structure of which serves as a hallmark signature of disorder. Through analytical and numerical modeling, we show that these fingerprints not only enable us to quantify minute amounts of disorder but also reveal its nature-as dilute vacancies. Remarkably, this model explains all details of the thermomagnetic response of our system, including the existence of multiple plateaus. Our findings provide a new approach to identifying disorder in quantum magnets.

We simulate the dynamical spin structure factor (DSSF) S(q, omega) of the spin-1/2 Heisenberg antiferromagnetic chain using classical simulations. By employing Landau-Lifshitz Dynamics, we emulate quantum correlations through temperature-dependent corrections, including rescaling of magnetic dipoles and renormalization of exchange interactions. Our results closely match Quantum Monte-Carlo calculations for kBT/J >= 1, extending the applicability of classical dynamics to the challenging case of gapless excitations. At higher temperatures, our simulations comply with general predictions for uncorrelated paramagnetic fluctuations in the infinite temperature limit. Entanglement witnesses derived from the quantum-equivalent DSSF act as sensitive diagnostics for the quantum-to-classical crossover. Their reliability stems from their dependence on spectral features alone, enabling classical dynamics to emulate quantum thresholds without genuine entanglement. This framework also reproduces transverse spin correlations in finite magnetic fields, in agreement with quantum simulations. Together, our results establish quantum-corrected classical dynamics as a scalable and predictive tool for interpreting scattering experiments and exploring quantum correlations in strongly correlated spin systems.

Multi-Q magnetic structures on two-dimensional (2D) lattices provide a key route to realizing topological physics in 2D magnetism. A major experimental challenge is to unambiguously confirm their formation by excluding the possibility of topologically trivial multidomain single- or double-Q magnetic orders, which cannot be distinguished using conventional diffraction techniques. Here, we propose that long-wavelength spin dynamics offers a universal diagnostic for triangular lattices: Triple-Q orders that preserve rotational symmetry and single- or double-Q orders that break it exhibit qualitatively distinct anisotropies in their Goldstone-mode velocities, stemming from fundamental differences in their underlying spin configurations. We validate this concept using the metallic triangular-lattice antiferromagnet Co_{0.325}TaS_{2}, which hosts both a stripe-type single-Q state and a triple-Q tetrahedral ordering at different temperatures. Using inelastic neutron-scattering and spin dynamics simulations, we first refine the spin Hamiltonian by fitting the paramagnetic excitation spectra, allowing us to develop an unbiased model independent of magnetic ordering. We then show that the observed velocity profiles of the Goldstone modes agree with the high-temperature model’s predictions: markedly anisotropic for the single-Q phase and near isotropic for the triple-Q phase. Importantly, this contrast persists across various exchange parameters, highlighting its model-independent nature and suggesting potential applicability to other 2D lattice systems. Beyond the long-wavelength regime, we present a substantial discrepancy between the measured and simulated magnon spectra exclusively in the triple-Q phase. We attribute this discrepancy to magnon energy renormalization arising from order-of-magnitude-enhanced magnon-magnon interactions in the triple-Q phase, due to its noncollinear configuration. This work provides universal insight into the dynamical properties of topological multi-Q magnetic orderings in 2D lattice structures, offering a broadly applicable diagnostic to distinguishing them from topologically trivial single- or double-Q counterparts. The unequivocal confirmation of the triple-Q structure in Co_{0.325}TaS_{2} further establishes it as a prominent material platform for exploring topological spin textures in the genuine 2D limit.

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.

Identifying the exotic quantum spin liquid phase in Kitaev magnets has garnered great research interests and remains a significant challenge. In experiments, most of the proposed candidate materials exhibit an antiferromagnetic (AFM) order at low temperatures, thus the challenge transforms into the searching for a field-driven disordered phase that is distinct from the partially polarized paramagnetic phase after suppressing the AFM order. Recently, Na$_2$Co$_2$TeO$_6$ has been proposed as one of the prime candidates, where the Kitaev interaction is realized by the high-spin $t^{5}_{2g}e^2_g$ configuration, and spin-orbit entangled $J_{\rm eff} = 1/2$ state in a bond-edge shared honeycomb lattice. In this study, we identify an emergent intermediate disordered phase induced by an external field along the $c$-axis of the honeycomb plane. This phase is characterized through magnetization and magnetocaloric effect experiments in high magnetic fields. To explain the experimental results, we propose an effective spin model with large AFM Kitaev interaction, which yields results in good agreement with both our findings and previously reported data. We determine that the effective $K$-$J$-$\Gamma$-$\Gamma'$ model for Na$_2$Co$_2$TeO$_6$ is nearly dual to that of $\alpha$-RuCl$_3$ under an unitary transformation. Given the insignificant fragility of Na$_2$Co$_2$TeO$_6$ sample, further high-field experiments can be conducted to explore this intermediate-field quantum spin disordered phase.

Spontaneous Hall conductivity has recently been reported in the triangular lattice antiferromagnet Co1/3TaS2 under a zero magnetic field. This phenomenon originates from the distinctive noncoplanar triple -Q magnetic ground state, possessing uniform real -space Berry curvature characterized by scalar spin chirality. We investigated the physical properties of Co1/3TaS2 by judiciously controlling the composition, revealing a drastic change in its bulk properties, even by slight variations in cobalt composition, despite the same crystal structure. For 0.299 <= x <= 0.325, CoxTaS2 keeps all the characteristics of the ground state consistent with the previous studies-two antiferromagnetic phase transitions at T-N1 and T-N2 (< T-N1), a large spontaneous Hall conductivity [sigma(xy)(H = 0)], and a weak ferromagnetic moment along the c axis. However, samples with x >= 0.330 exhibit distinct bulk properties, including the absence of both sigma xy(H = 0) and the weak ferromagnetic moment. Our neutron diffraction data reveal that CoxTaS2 with x >= 0.330 develops coplanar helical magnetic order with q(m1) =(1/3, 0, 0). This is entirely different from what has been seen in x <= 0.325, explaining the observed composition dependence.

TM1/3MS2 (TM = 3d transitionmetal, M = Nb, Ta) has recently attracted increasing attention due to its wide variety of fascinating magnetic structures and the chiral arrangement of intercalated TM atoms. We investigated the bulk properties of Ni1/3NbS2 and Ni1/3TaS2 using magnetization, transport, heat capacity, powder neutron diffraction, and x-ray absorption spectroscopy. Ni1/3NbS2 undergoes a phase transition at 84 K, developing an antiferromagnetic helical order with a very long period along the c axis (33c). On the other hand, a simple A-type spin configuration was observed for Ni1/3NbS2 below 158 K, where the spins are aligned to the c axis. These magnetic structures, combined with lattice chirality and metallicity, can lead to various intriguing transport properties, making Ni1/3NbS2 and Ni1/3TaS2 promising material candidates for future studies on antiferromagnetic spintronics.

We report on our terahertz spectroscopic investigation of the van der Waals insulator NiI2, exhibiting antiferromagnetism below T-N1 similar or equal to 78 K and multiferroicity below T-N2 similar or equal to 59.5 K. Two electromagnon modes were detected at 34 and 37 cm(-1) below TN2 where the material is in the helimagnetic-multiferroic phase. Our transmission measurement shows that the electromagnon resonance modes redshift with increasing temperature (at zero magnetic field) but blueshift with increasing magnetic field (at 1.5 K). A separate reflection measurement confirms the electric dipole active nature of the two electromagnon modes. The polarization, temperature, and magnetic field dependences show that these electromagnon modes are closely linked with the helimagnetic ordering in the multiferroic phase of NiI2. The electromagnon energies are also consistent with the energy scale of the two-magnon sideband excitation around the Zhang-Rice exciton mode recently discovered in NiI2.

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. Geometric frustration and bond-dependent interactions each introduce quantum fluctuations that can create spin liquid phases. Now it is shown that CoI2 is a triangular lattice material that combines both.

$T{M}_{1/3}M{S}_{2}$ ($TM=3d$ transition metal, $M=\mathrm{Nb},\phantom{\rule{0.16em}{0ex}}\mathrm{Ta}$) has recently attracted increasing attention due to its wide variety of fascinating magnetic structures and the chiral arrangement of intercalated $\mathit{TM}$ atoms. We investigated the bulk properties of ${\mathrm{Ni}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$ and ${\mathrm{Ni}}_{1/3}\mathrm{Ta}{\mathrm{S}}_{2}$ using magnetization, transport, heat capacity, powder neutron diffraction, and x-ray absorption spectroscopy. ${\mathrm{Ni}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$ undergoes a phase transition at 84 K, developing an antiferromagnetic helical order with a very long period along the $c$ axis ($33c$). On the other hand, a simple $A$-type spin configuration was observed for ${\mathrm{Ni}}_{1/3}\mathrm{Ta}{\mathrm{S}}_{2}$ below 158 K, where the spins are aligned to the $c$ axis. These magnetic structures, combined with lattice chirality and metallicity, can lead to various intriguing transport properties, making ${\mathrm{Ni}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$ and ${\mathrm{Ni}}_{1/3}\mathrm{Ta}{\mathrm{S}}_{2}$ promising material candidates for future studies on antiferromagnetic spintronics.