The system at the heart of the study is the thiaporphyrin, a porphyrin variant in which one pyrrole ring of the macrocycle is replaced by a thiophene. Porphyrins are among the most versatile ligands in coordination chemistry, and the thiaporphyrin framework brings an additional twist: as a monoanionic ligand, it only partially compensates the charge of a divalent metal ion. When a divalent metal such as copper is complexed, the resulting π-electronic complex remains a cation, and it therefore must pair with an anion in any isolable salt. This charge imbalance, usually treated as a chemical bookkeeping detail, is precisely what the Ritsumeikan team exploited. By choosing different counteranions, they could rewrite the electrostatic landscape of the crystal without touching the copper center that carries the spin.

Incorporating copper(II) is what makes the strategy magnetically meaningful. The Cu(II) ion contributes a single unpaired electron, so each complex cation is simultaneously a charged building block and a paramagnetic unit. Whether two such units interact magnetically depends on how closely their spin-bearing cores approach and how those cores are oriented relative to one another. Intermolecular spin–spin interactions in Cu(II) complexes of π-electronic macrocycles had previously been reported in only a limited number of cases, which left open a fundamental question: could the assembly mode of these ion pairs be deliberately steered, and would the steering translate into measurable changes in magnetic behavior?

To answer that question, the researchers synthesized two thiaporphyrin Cu(II) complex cations and initially isolated them as chloride salts. They then exchanged the chloride counterions for a chemically diverse panel of anions: tetrafluoroborate (BF4−), hexafluorophosphate (PF6−), tetrakis(pentafluorophenyl)borate (B(C6F5)4−, abbreviated FABA−), and pentacyanocyclopentadienide (PCCp−). This set spans a wide range of shapes and electronic characters, from compact, roughly spherical inorganic anions to a large, weakly coordinating borate and a flat, π-electronic cyclopentadienide derivative decorated with five cyano groups. Each ion pair was then interrogated with a combination of single-crystal X-ray analysis, solid-state electron spin resonance (ESR) spectroscopy, magnetic susceptibility measurements, UV/visible spectroscopy, and theoretical calculations, allowing the team to connect crystallographic structure directly to magnetic response.

The structural contrast that emerged was striking. With the π-electronic PCCp− counteranion, one of the Cu(II) complexes formed a charge-by-charge assembly, in which cations and anions alternate in π-stacked ion pairs, each positive unit sandwiched against a negative partner. A second PCCp− ion pair adopted a different motif, featuring axial Cu–N coordination involving the anion. Critically, in the charge-by-charge arrangement the spin density remained largely localized on the CuN3S core of the cation, with negligible delocalization onto the PCCp− anion. Because the paramagnetic centers were effectively insulated from one another by the interleaved anions, no significant intermolecular spin–spin interactions were detected. The alternating stack, elegant as a supramolecular architecture, is magnetically quiet.

The nonplanar counteranions told a very different story. Ion pairs containing BF4−, PF6−, or FABA− did not interleave with the cations in the same way. Instead, the complex cations stacked face-to-face with one another, forming π-stacked cation dimers that further assembled in a two-by-two packing mode. Here the anions occupy the space around the dimers rather than between the stacked cations, and the two spin-bearing CuN3S cores are brought into close, well-defined proximity. ESR and magnetic susceptibility measurements on these dimer-based structures indicated antiferromagnetic interactions, meaning the neighboring spins couple in an opposing alignment that suppresses the net magnetic moment. Theoretical spin-density calculations supported the experimental observations, showing opposite spins localized on the respective stacked Cu(II)-containing cations, exactly the picture expected for an antiferromagnetically coupled dimer.

Perhaps most instructively, the study showed that even among the dimer-forming salts, the strength of the antiferromagnetic interaction was not uniform. Differences in local S/N contacts between stacked cations and differences in how the dimers packed against one another were associated with differences in the magnitude of the coupling. In other words, the counteranion influenced not only whether dimerization occurred at all, but also the fine geometry of the dimers and their higher-order arrangement in the crystal. The distance and orientation of the CuN3S units proved to be the crucial factors governing spin–spin coupling, while chalcogen-bonding and dipole–dipole interactions helped stabilize the stacked dimers. Magnetism, in this system, is a readout of packing geometry at multiple hierarchical levels.

From a materials-design standpoint, the implications reach beyond porphyrin chemistry. Spintronics, the exploitation of electron spin alongside charge in devices, has long sought molecular platforms in which magnetic interactions can be tuned rationally. Conventional approaches modify the ligand or the metal to adjust magnetic coupling, but the Ritsumeikan strategy operates one level higher: it leaves the paramagnetic unit untouched and rewrites the assembly script through ion pairing. Because counteranion exchange is a comparatively mild post-synthetic operation, the same complex cation could in principle be delivered into crystals with different magnetic characters simply by changing the salt it is crystallized from. As Professor Maeda noted, the Cu(II) complexation of thiaporphyrins affords paramagnetic π-electronic cations that modulate ion-pairing assembly modes in combination with coexisting anions, and the design of π-electronic systems with charge and spin would provide fascinating strategies for the construction of supramolecular spintronic materials.

The study also refines the conceptual vocabulary of charged π-electronic systems. Oppositely charged species tend to form charge-by-charge assemblies driven by electrostatic attraction, whereas like-charged π-electronic units can, under favorable intermolecular interactions, overcome electrostatic repulsion and form stacked dimers. Which of these competing tendencies wins depends on a delicate balance of electrostatics, dispersion forces, and directional secondary interactions such as chalcogen bonding. By mapping how four distinct anions tip that balance in a single family of complexes, the researchers have turned a qualitative intuition into a set of experimentally grounded structural criteria: keep the spin cores apart and the material is magnetically silent; bring them into stacked proximity with the right contacts and antiferromagnetic coupling appears.

What remains to be explored is how far this ion-pairing lever can be pushed. The findings demonstrate that counteranions can direct the assembly of paramagnetic molecular cations and, in turn, modulate their collective magnetic properties, but extending the approach to ferromagnetic coupling, to switchable or responsive materials, and to processable thin films will require further work. Even so, the message of the study is clear and broadly applicable: in charged π-electronic systems, the counterion is not passive baggage but an architectural agent. By using ion pairing to control dimerization and spin arrangements, chemists now have a molecular design strategy for constructing supramolecular spintronic materials in which magnetic behavior can be tuned through assembly, one carefully chosen counteranion at a time.

Subject of Research: Counteranion-controlled ion-pairing assembly and magnetic properties of thiaporphyrin Cu(II) complex cations

Article Title: Counteranions reshape molecular packing to tune magnetism

Article References: Counteranions reshape molecular packing to tune magnetism. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: thiaporphyrin, copper(II) complexes, counteranions, ion pairing, antiferromagnetism, π-stacking, supramolecular chemistry, spintronics, molecular magnetism, chalcogen bonding, crystal packing, Chemical Science

A new numerical study published in Nature Physics by Simone Fratini, Ivan Duchemin, Arnaud Ralko and Sergio Ciuchi now offers a strikingly simple answer. Working within the two-dimensional t–J model, the canonical minimal model of doped Mott insulators used to describe cuprate superconductors, the team shows that strange metallicity is not an exotic phase requiring special ingredients. Instead, it emerges pervasively across the temperature-doping phase diagram whenever antiferromagnetic order is suppressed. The strange metal, in their results, is the natural descendant of the bad metal that underlies it, and its hallmark properties follow from the quantum statistics of the charge carriers rather than from any special temperature dependence of the processes that scatter them.

The bad metal concept, introduced in the early 2000s, describes a regime in which the electrical resistivity exceeds the Mott–Ioffe–Regel limit, the value at which an electron’s mean free path becomes comparable to its own wavelength. In such a regime, the very notion of a well-defined quasiparticle travelling between collisions breaks down, and transport must be understood through more general principles of quantum diffusion. Bad metallicity had been observed in numerical studies of doped Mott insulators for years, but its relationship to the strange metal phenomenology of the cuprates, with its linear resistivity and Planckian dissipation, remained ambiguous. The new work argues that the two are, in a precise sense, the same phenomenon viewed at different levels of description.

The technical achievement that makes this conclusion possible lies in recent improvements to the finite-temperature Lanczos method, a numerically exact technique for computing the thermodynamic properties of strongly correlated lattice models at finite temperature. The Lanczos approach works by iteratively building a small set of basis vectors that span the low-energy part of the Hilbert space, allowing accurate evaluation of correlation functions without ever handling the exponentially large full space. Recent methodological advances, building on decades of development since the original finite-temperature Lanczos work of the 1990s, now enable exact calculations at low temperatures and with high spectral resolution, precisely the regime where strange metal behaviour lives and where earlier approximations faltered.

Using this machinery, the researchers computed both the direct-current and the frequency-dependent conductivity of the two-dimensional t–J model across a broad range of temperatures and dopings. The t–J model describes holes moving through an antiferromagnetic background of spin-one-half particles, with a superexchange coupling J between neighbouring spins and a hopping amplitude t that is strongly constrained by the no-double-occupancy condition. This constraint, a remnant of the large on-site repulsion of the underlying Hubbard model, is what makes the model so difficult and so rich. It is also, according to the new results, what makes it strange: the interplay of quantum statistics and the constrained motion of carriers produces transport that looks Planckian without requiring any exotic scattering mechanism.

A central question in the strange metal literature concerns the origin of Planckian relaxation, the observation that the scattering time in these materials is of order ħ divided by the Boltzmann constant times temperature, the fastest timescale that quantum mechanics permits for a thermal system. Some theorists have proposed that Planckian behaviour signals proximity to a quantum critical point, where fluctuations occur at all scales and times. Others have pointed to spatially random interactions, to spin fluctuations, or to specific scattering channels as the culprit. The new study addresses this question directly by extending the analysis into the frequency and time domains, where the dynamics of the charge response can be watched as it unfolds rather than merely inferred from the final resistivity.

What emerges from this time-resolved picture is that Planckian behaviour is rooted in the quantum statistical nature of the charge response itself. The carriers in the doped antiferromagnet are incoherent from the outset, and their transport is governed by the statistical mechanics of a dense quantum fluid rather than by the collision history of individual particles. The temperature dependence of the scattering processes, which in conventional thinking would determine the resistivity, turns out to play a secondary role. In effect, the strange metal does not become strange because something unusual starts scattering its electrons at high temperatures; it is strange because the carriers were never the well-behaved quasiparticles of Fermi liquid theory in the first place.

This reframing has significant implications for how physicists interpret experiments on the cuprates and related materials. The linear-in-temperature resistivity observed over wide doping ranges in compounds such as La2−xSrxCuO4 and Bi-based cuprates has often been treated as a fingerprint of a specific mechanism, to be isolated and identified. If the new numerical results are a faithful guide, the linear resistivity instead reflects a generic property of doped Mott insulators once long-range magnetic order gives way to a quantum paramagnet. The experimental phase diagrams of the cuprates, in which the strange metal region tracks the suppression of antiferromagnetism and the pseudogap boundary, are consistent with this picture, and recent measurements of spin-charge correlations at the onset of the pseudogap add further context for testing it.

The study also connects to a broader theoretical programme aimed at understanding transport in systems where quasiparticles fail. Previous work on the Hubbard model, on extremely correlated Fermi liquid theory, and on random-interaction models of strange metals has each highlighted different aspects of non-quasiparticle transport. By demonstrating that a single, well-defined minimal model reproduces the full phenomenology, from bad metallicity at high temperatures to Planckian strange metal behaviour at lower temperatures, the new results provide a unifying numerical benchmark against which analytical theories can be tested. The authors have made both their data and their code publicly available on GitHub, lowering the barrier for other groups to verify and extend the calculations.

For the field of quantum materials, the message is both humbling and clarifying. The strange metal, the state that parents high-temperature superconductivity and has resisted explanation since the discovery of the cuprates in 1986, may not require new particles, hidden criticality, or exotic interactions. It may simply be what a strongly correlated quantum fluid looks like when the usual quasiparticle picture is abandoned and the quantum statistics of the carriers are taken seriously. If further work confirms that the t–J model captures the essential physics, the path toward a complete theory of the cuprates becomes clearer: understand the constrained quantum statistics of doped antiferromagnets, and the strange metal, along with perhaps the superconductivity that emerges from it, will follow. The puzzle is far from solved, but the new results suggest that the answer may have been hiding in one of the oldest and simplest models in the theorist’s toolkit.

Subject of Research: The microscopic origin of strange metal behaviour and Planckian transport in doped Mott insulators

Article Title: Strange metal behaviour from underlying bad metallicity

Article References: Fratini, S., Duchemin, I., Ralko, A., & Ciuchi, S. (2026). Strange metal behaviour from underlying bad metallicity. Nature Physics. https://doi.org/10.1038/s41567-026-03441-x

Image Credits: AI Generated

DOI: 10.1038/s41567-026-03441-x

Keywords: strange metal, bad metal, Planckian dissipation, t–J model, cuprates, high-temperature superconductivity, Mott insulator, finite-temperature Lanczos method, quantum statistics, antiferromagnetism, electrical resistivity, strongly correlated electrons

To appreciate why this matters, it helps to understand what makes kagome lattices so special. The kagome pattern is a two-dimensional tiling of corner-sharing triangles, and when magnetic atoms such as manganese arrange themselves on such a lattice, the resulting electronic structure is anything but ordinary. Quantum mechanics dictates that electrons moving through this geometry encounter Dirac cones, where bands cross linearly like in graphene; flat bands, where electrons become effectively immobile; and van Hove singularities, where the density of available electronic states spikes dramatically. These features concentrate an abstract quantum-mechanical quantity called Berry curvature into sharp hotspots in momentum space, and Berry curvature is the engine that drives the anomalous Hall effect, the appearance of a transverse voltage in a magnetic material even without an external Lorentz force.

YMn6Sn6 has become a poster child for this class of materials. It crystallizes in a hexagonal structure in which planes of manganese atoms form the kagome lattice, sandwiched between layers of tin and mixed yttrium-tin. The compound orders antiferromagnetically below a remarkably high Néel temperature of about 345 kelvin, comfortably above room temperature, and its low-temperature ground state is an incommensurate distorted spiral of manganese moments. When researchers apply a magnetic field within the kagome plane, this spiral unwinds in stages: first into a transverse conical spiral, then a fan-like configuration, then an intermediate phase, and finally a forced ferromagnetic state in which all moments align with the field. Magnetization measurements on the team’s single crystals revealed sharp anomalies at fields of roughly 2.5, 7.0, 9.9, and 10.5 tesla, mapping out this rich sequence of magnetic phases with excellent agreement to earlier work.

The crystals themselves are a triumph of materials synthesis. Grown from a tin flux, they exhibit residual resistivity ratios of about 44 along the crystallographic c-axis and 30 within the kagome plane, placing them squarely in what physicists call the clean transport regime. At low temperatures the electrical conductivity approaches 10^6 siemens per centimeter, comparable to conventional ferromagnetic metals at their purest. This cleanliness turns out to be far more than a technical achievement; as the new study demonstrates, it is the enabling condition for the record-breaking physics that follows, because certain scattering mechanisms that amplify the Hall response only become dominant when impurity densities are vanishingly small.

To connect the magnetic phases to electronic structure, the team combined three complementary probes. Angle-resolved photoemission spectroscopy, performed at the Pohang Light Source and the Advanced Light Source, directly imaged the bands of electrons near the Fermi level, revealing Dirac-like dispersions near the K points of the Brillouin zone and a flat band roughly 0.3 to 0.4 electron-volts below the Fermi energy. These measured dispersions matched density functional theory calculations remarkably well, provided the theorists modeled the spiral order with a commensurate double-layer antiferromagnetic configuration in which manganese moments are ferromagnetically aligned within each kagome bilayer but antiferromagnetically coupled between bilayers. Orbital analysis showed that the Dirac bands near the Fermi level derive mainly from manganese dz2 orbitals, while the flat band is dominated by in-plane dxy and dx2-y2 orbitals, a signature of the multi-orbital character of kagome electronics.

The theoretical centerpiece of the study is the demonstration that spin canting systematically rewrites this electronic landscape. The researchers modeled the effect of an in-plane field by rotating the manganese moments away from their antiferromagnetic orientation by an angle theta, sweeping continuously from the antiferromagnetic state at zero degrees to full ferromagnetic alignment at ninety degrees. As theta increases, the kagome flat bands spin-split and slide downward toward the Fermi level, while the band folding associated with the doubled unit cell weakens and eventually disappears. Most strikingly, at canting angles near seventy degrees, a spin-orbit-coupling-induced gap opens along the M-L direction of the Brillouin zone, and enormous Berry curvature hotspots erupt near the M point. The calculated intrinsic anomalous Hall conductivity peaks at exactly this canting angle, a robust result that survives even if the Fermi level is shifted by fifty milli-electron-volts.

The experimental transport data bear this prediction out with uncanny fidelity. After carefully subtracting the ordinary Hall contribution, which arises from the Lorentz force on moving charges, the team isolated the anomalous Hall conductivity as a function of field. For fields applied within the kagome plane, the anomalous component σxzA climbs through the sequence of magnetic phases and reaches a staggering maximum of roughly 2.5 x 10^4 siemens per centimeter at 9.5 tesla, right in the fan-like phase and precisely where the calculations place the Berry curvature maximum. Above the saturation field of about 13.5 tesla, in the fully polarized ferromagnetic state, the value settles near 5.0 x 10^3 siemens per centimeter, still far above the best ferromagnets on record, including nickel at about 1.6 x 10^3, Co2MnGa at about 2.0 x 10^3, iron-doped CoS2 at about 2.5 x 10^3, and NdCrSb3 at about 2.9 x 10^3 siemens per centimeter.

What makes these numbers truly extraordinary is that they cannot be explained by the standard intrinsic mechanism alone. The intrinsic Berry-curvature contribution to anomalous Hall conductivity is bounded by a fundamental quantum scale of roughly e2 per hectare, about 10^3 siemens per centimeter, and the calculated intrinsic value for the ferromagnetic phase of YMn6Sn6 falls well short of what was measured. The measured conductivity exceeds the calculation by a factor of about twenty-three, an enhancement ratio an order of magnitude larger than the factor of one to four seen in other colossal anomalous Hall materials. Conventional skew scattering, in which spin-orbit coupling at impurity sites bends electron trajectories asymmetrically, also falls short of explaining such a large amplification.

The resolution, the authors argue, lies in a more powerful mechanism: Berry-curvature-induced skew scattering. Recent theoretical work has shown that finite Berry curvature renders electron wavefunctions noncommutative in real space, producing asymmetric scattering probabilities even when the impurities themselves carry no spin-orbit coupling. The asymmetric scattering rate in this framework scales with the impurity potential cubed times the Berry curvature at the Fermi wavevector, whereas conventional impurity skew scattering scales only linearly with impurity spin-orbit strength. Plugging in realistic numbers, the Berry-curvature-driven channel can outperform the conventional one by one to two orders of magnitude, exactly the amplification needed to account for the colossal values observed in the clean crystals of YMn6Sn6. Below saturation, the same mechanism, fed by the strongly field-tunable Berry curvature of the canted spin phases, reproduces the observed peak near 9.5 tesla when the same enhancement factor is applied to the calculated curve.

Perhaps the most consequential implication is conceptual. For decades, physicists have assumed that the anomalous Hall conductivity should simply track the magnetization, so that any deviation is attributed to a topological Hall effect arising from scalar spin chirality in real space. Earlier work on YMn6Sn6 had indeed invoked such a topological contribution from its spiral textures. But the new data argue against that interpretation: the Hall response peaks in the fan-like phase, where the spin configuration subtends negligible solid angle and chirality should vanish, while the transverse conical phase, where neutron experiments confirm sizable chirality, shows no clear topological Hall signature at low temperatures. Moreover, for fields along the c-axis, where moments cant continuously without phase transitions, the anomalous Hall conductivity grows monotonically all the way into the chirality-free ferromagnetic state. The message is clear: in kagome antiferromagnets, flat bands and Dirac crossings near the Fermi level act as such potent reservoirs of momentum-space Berry curvature that even gentle field-induced spin canting can dramatically reshape the Hall response, and in clean crystals this tunable curvature is amplified to record heights. For engineers dreaming of electrically reconfigurable topological devices, YMn6Sn6 just became the most exciting material on the bench.

Subject of Research: Magnetic-field-induced Berry curvature modulation and colossal anomalous Hall conductivity in the kagome antiferromagnet YMn6Sn6

Article Title: Colossal Magnetic‐Field‐Induced Anomalous Hall Conductivity in a Kagome Antiferromagnet YMn6Sn6

Article References: Choi, M., Kim, H., Sung, M., Seo, H., Choi, J., Kim, J., Jo, Y., Yeom, H. W., Jin, K.-H., & Kim, J. S. (2026). Colossal Magnetic‐Field‐Induced Anomalous Hall Conductivity in a Kagome Antiferromagnet YMn 6 Sn 6. Advanced Science, Article e77738. https://doi.org/10.1002/advs.77738

Image Credits: AI Generated

DOI: 10.1002/advs.77738

Keywords: kagome lattice, YMn6Sn6, antiferromagnetism, anomalous Hall effect, Berry curvature, spin canting, skew scattering, Dirac bands, flat bands, spintronics, ARPES, density functional theory

Writing in Nature Materials, Pratap Chandra Adak, Vinod M. Menon and colleagues demonstrate coherent microwave-to-optical transduction in chromium sulfide bromide, or CrSBr, a van der Waals antiferromagnet that has quickly become one of the most studied materials in condensed matter physics. The work, performed in collaboration with researchers at the CUNY Advanced Science Research Center, Columbia University, the University of Chemistry and Technology Prague, the University of Chicago and Rheinland-Pfälzische Technische Universität Kaiserslautern, shows that the magnetic and optical excitations of CrSBr are coupled strongly enough that a microwave signal applied to the crystal is faithfully written onto an optical beam reflected from its surface. Remarkably, the demonstration required no optical cavity, no cryogenic microwave resonator and no mechanical oscillator.

The central trick lies in the material’s excitons, tightly bound electron-hole pairs that dominate the optical response of CrSBr. In this layered magnet, excitons are not passive spectators; they interact intimately with the ordered array of chromium spins that gives the crystal its antiferromagnetic character. Previous studies had shown that coherent spin waves, known as magnons, can shift and modulate exciton resonances in van der Waals magnets, and that exciton-polaritons, hybrid light-matter quasiparticles, can be magnetically dressed in CrSBr. The New York-led team turned this magnon-exciton dialogue into an engineering resource: by driving the antiferromagnetic resonance of the crystal with microwaves, they modulate the resonant excitonic susceptibility, effectively making the material’s optical properties oscillate at the microwave frequency.

That oscillating susceptibility has a precise consequence in the language of nonlinear optics. When a continuous laser beam impinges on the crystal, the microwave-driven modulation scatters light into sidebands shifted up and down in frequency by exactly the microwave frequency, a process analogous to frequency conversion in an electro-optic modulator but mediated by collective spin dynamics rather than an applied electric field. The team detected these sidebands using homodyne interferometry, a phase-sensitive technique that mixes the converted light with a reference beam derived from the same laser. Because homodyne detection preserves phase information, it verifies that the conversion is coherent, meaning the quantum state of a microwave excitation would, in principle, be preserved as it crosses the frequency divide, an essential requirement for quantum networking applications.

The breadth of the conversion window is one of the most notable technical results. Many cavity-based transducers operate over narrow bandwidths, trading conversion efficiency against the range of microwave frequencies they can handle, a limitation that complicates interfacing with real superconducting qubits whose transition frequencies vary from device to device. The CrSBr platform exhibited coherent conversion over an intrinsically broadband window of roughly 300 megahertz, even in a plain bulk crystal with no cavity enhancement of any kind. This bandwidth arises naturally from the magnon-exciton coupling mechanism rather than from careful impedance engineering, suggesting it can be retained, and potentially widened, as devices shrink and become more sophisticated.

Equally significant is what the team observed when they tuned the probe laser away from the bare exciton resonance. Multiple exciton-polariton resonances, the hybrid modes that emerge when excitons couple strongly to photonic modes, all inherited the magnon-coupled response. In other words, the spin-driven transduction is not confined to a single narrow spectral line; it persists across several polariton branches. This inheritance points to a strategy for broadening the usable optical detuning range and for mitigating optical dissipation, since different polariton resonances offer different trade-offs between light-matter coupling strength and absorption loss. It also hints that polariton engineering, a specialty of the Menon laboratory, could become a design tool for future transducers rather than an afterthought.

The choice of material is doing heavy lifting here. CrSBr is an easy-axis van der Waals antiferromagnet whose gigahertz antiferromagnetic resonances sit squarely in the microwave band relevant to superconducting circuits. Unlike ferromagnetic insulators used in earlier magnon-based conversion proposals, CrSBr is also a semiconductor with sharp, strongly absorbing exciton resonances, so the magneto-optical interaction is not limited to the intrinsically weak, off-resonant Faraday-type effects that have historically hampered magnon transducers. Instead, the conversion leverages strong light-matter interactions at the exciton resonance, where even modest spin-driven changes in the dielectric function translate into large changes in optical phase and amplitude. The layered crystal structure is an additional bonus: CrSBr can be exfoliated to atomically thin flakes, and its magnetic order survives down to the two-dimensional limit, making the platform a natural candidate for integration into nanoscale photonic and spintronic circuits.

The authors are candid that the current demonstration is a proof of principle rather than a finished quantum link. The transduction efficiency observed in bulk crystals remains far from the unity conversion needed for lossless quantum state transfer, and the measurements were performed without the cavity enhancement that defines state-of-the-art electro-optic and optomechanical transducers. But the pathway to higher performance is spelled out by the physics itself: the transduction strength scales with the cooperativity between magnons and excitons, which can be increased by reducing the magnetic volume of the device and by adding optical cavities that lengthen the interaction time between light and the exciton-polariton modes. Shrinking the crystal to a nanoscale flake on a photonic chip, while keeping the microwave field concentrated, would boost the coupling per photon dramatically. Cavity integration would additionally sharpen the collection efficiency and could enable operation in the quantum-coherent regime where single microwave photons are converted into single optical photons.

If those engineering steps succeed, the implications extend across the quantum technology landscape. Superconducting quantum processors, arguably the leading platform for scalable quantum computing, produce and manipulate quantum information in the microwave domain but cannot easily communicate with one another over distance, since microwave photons are hopelessly lossy in fiber and even in free space at room temperature. A compact, broadband, fiber-compatible transducer would allow these processors to be linked into distributed networks, enabling modular quantum computing and long-baseline quantum sensing. Magnetic memory and spintronic devices, which operate naturally at microwave frequencies, could likewise be networked through optical links. Because CrSBr transduction is broadband, a single device could conceivably serve several qubits operating at slightly different frequencies, simplifying system architectures considerably.

The work also adds momentum to a broader renaissance in magnon-based quantum technologies. Theorists have recently proposed antiferromagnet-based microwave-to-optical quantum transduction schemes, and experimentalists have demonstrated coherent magnon-photon coupling in CrSBr itself, as well as magnon-mediated interactions between excitons. The new results tie these threads together into a concrete transduction demonstration, showing that a single van der Waals crystal can host the magnetic, electronic and optical degrees of freedom needed for frequency conversion simultaneously. For a field accustomed to stitching together transducers from superconducting circuits, piezoelectric resonators, electro-optic crystals and atomic vapors, the appeal of a monolithic, two-dimensional magnetic semiconductor that does the whole job is obvious. The CrSBr crystal sitting on the researchers’ microwave waveguide is, in effect, an entire quantum translator etched by nature down to a few atomic layers, and the race is now on to see how far this elegantly simple platform can be pushed.

Subject of Research: Coherent broadband microwave-to-optical quantum transduction via magnon-exciton coupling in the layered antiferromagnet CrSBr

Article Title: Microwave-to-optical transduction using magnon–exciton coupling

Article References: Adak, P. C., McDaniel, I. E., Paul, S., Heuvel-Horwitz, C., Das, B., Kozlov, V., Mosina, K., Ramanathan, A., Roy, X., Sofer, Z., Zhong, T., Kamra, A., Thielens, A., Alú, A., & Menon, V. M. (2026). Microwave-to-optical transduction using magnon–exciton coupling. Nature Materials. https://doi.org/10.1038/s41563-026-02748-7

Image Credits: AI Generated

DOI: 10.1038/s41563-026-02748-7

Keywords: CrSBr, microwave-to-optical transduction, magnon-exciton coupling, quantum networks, exciton-polaritons, antiferromagnetism, van der Waals magnets, magneto-optics, spintronics, quantum transducers, two-dimensional materials, homodyne detection

The material at the heart of the study is chromium thiophosphate, CrPS4, an air-stable, layered magnetic semiconductor that has become a workhorse of two-dimensional magnetism research. On its own, CrPS4 behaves conventionally depending on thickness: few-layer samples can show ferromagnetic order, while bulk crystals adopt an antiferromagnetic arrangement. Neither of these ground states, however, exhibits the hallmark of altermagnetism, which is a finite splitting between electronic states of opposite spin that does not arise from spin-orbit coupling but instead from the symmetry of the crystal and its magnetic arrangement. The researchers reasoned that if two CrPS4 flakes were stacked with their crystal axes rotated by ninety degrees, the interlayer symmetry would be altered in precisely the way theory predicts should generate an altermagnetic state.

The idea traces back to theoretical work suggesting that twisted magnetic van der Waals bilayers constitute an ideal platform for altermagnetism. In an ordinary antiferromagnet, time-reversal symmetry combined with a lattice translation protects the degeneracy of spin-up and spin-down bands, so the electronic structure shows no net spin splitting. When layers are rotated relative to one another, that combined symmetry operation is broken, and the spin degeneracy is lifted in a momentum-dependent fashion. The result is a material whose spins cancel macroscopically, avoiding the stray fields that plague ferromagnetic devices, yet whose electronic bands are split in a way that can carry spin-polarized currents. For spintronics, this combination is extraordinarily attractive: fast switching, dense packing, and robust spin information without the crosstalk that limits conventional magnetic memory.

To test the prediction, the team fabricated orthogonally twisted CrPS4/CrPS4 homostructures, stacking four-layer flakes rotated by ninety degrees relative to each other. They then interrogated the samples with a battery of optical probes, beginning with polarized Raman spectroscopy. In the twisted configuration, the Raman spectra revealed a splitting of phonon modes that is absent in untwisted controls. This vibrational fingerprint, the researchers show, arises from the interlayer coupling unique to the altermagnetic arrangement: the same symmetry breaking that splits the electronic bands also modifies the lattice dynamics, producing a spectroscopic signature that distinguishes the twisted state from any simple superposition of ferromagnetic and antiferromagnetic layers.

The more decisive evidence came from magneto-optical spectroscopy. Measuring the photoluminescence of the twisted homostructure under an applied magnetic field, the researchers tracked the degree of circular polarization of the emitted light. The twisted sample displayed a magnetic-field dependence that resembles the response of a ferromagnet, a striking result given that the underlying spin order is compensated. Even more telling was the observation of a pronounced Zeeman splitting in the photoluminescence spectra, with the sigma-plus and sigma-minus emission peaks separating as the field increased. Crucially, neither ferromagnetic nor antiferromagnetic CrPS4 samples show this splitting in the same configuration. Its emergence only in the twisted geometry indicates that the stack has entered a genuinely distinct magnetic ground state, one that carries the fingerprints of altermagnetism.

First-principles calculations reinforced the interpretation. Using density functional theory with an on-site Coulomb correction to treat the correlated chromium d electrons, the team computed the electronic structure of the orthogonally twisted bilayer and found large spin-split bands in an antiferromagnetic configuration. The pattern of the splitting matches the theoretical expectations for altermagnets: opposite-spin bands separate in momentum space in a way dictated by crystal rotation symmetry, while the net magnetization remains zero. The agreement between the computed band structure and the optical measurements provides a coherent picture in which twisting acts as a symmetry-breaking knob that switches on spin splitting without introducing any net magnetic moment.

The significance of the result extends beyond confirming a theoretical prediction. Van der Waals homostructures, in which the same material is stacked with controlled twist angles, have already revolutionized research on graphene and transition metal dichalcogenides, giving rise to moiré physics, flat bands, and correlated electronic phases. The present work shows that the same stacking engineering can serve as a design principle for magnetism itself. Rather than searching for new chemical compounds that happen to be altermagnets, researchers can now, in principle, manufacture altermagnetic states from well-characterized magnetic layers by choosing the twist angle. Because the twist angle is a continuously tunable parameter, it opens a route to systematically exploring how altermagnetic spin splitting evolves with interlayer symmetry, something no fixed crystal structure can offer.

The experimental signatures reported here also add to a growing toolbox for identifying altermagnets. Earlier confirmations of altermagnetic band splitting relied on techniques such as spin-resolved and angle-resolved photoemission spectroscopy, x-ray magnetic circular dichroism, and magneto-optical Kerr effect measurements in compounds like CrSb, MnTe, and RuO2. The CrPS4 study demonstrates that circularly polarized photoluminescence and Raman spectroscopy, both accessible table-top optical methods, can detect altermagnetism in atomically thin devices. This accessibility matters: optical probes can be applied to microscopic samples inside cryostats, under magnetic fields, and across device geometries, accelerating the pace at which candidate altermagnetic structures can be screened and characterized.

For applications, the appeal of altermagnets lies in their potential to combine the speed and stability of antiferromagnetic spintronics with the readout convenience of ferromagnets. Antiferromagnetic memory elements are immune to external fields and can in principle switch at terahertz frequencies, but their vanishing net moment makes them hard to read. Altermagnets solve the readout problem because their spin-split bands allow spin-polarized transport and magneto-optical signals even without net magnetization. A twisted van der Waals altermagnet adds another dimension: the state exists in an atomically thin, air-stable semiconductor that can be integrated into heterostructures with other two-dimensional materials, potentially enabling spin filters, tunnel junctions, and valleytronic devices in which spin and momentum are locked by design.

Challenges remain before such devices materialize. The reported signatures are spectroscopic rather than transport-based, and future work will need to demonstrate electrical readout and manipulation of the altermagnetic state, quantify the magnitude and temperature stability of the spin splitting, and explore how different twist angles and layer numbers tune the effect. Still, the demonstration that a simple ninety-degree rotation of identical CrPS4 flakes produces a magnetic phase absent from either constituent marks a conceptual milestone. It establishes twisted van der Waals homostructures as an experimental platform for altermagnetism, transforming a theoretical proposal into a tangible, measurable state of matter and handing the spintronics community a new material class to engineer.

Subject of Research: Experimental observation of altermagnetism in orthogonally twisted CrPS4 van der Waals homostructures

Article Title: Altermagnetism in twisted van der Waals homostructures

Article References: Chen, J., Xie, X., Li, S., Zhang, S., Hou, S., Zhang, X., He, J., Liu, Z., Wang, J.-T., & Liu, Y. (2026). Altermagnetism in twisted van der Waals homostructures. Nature Physics. https://doi.org/10.1038/s41567-026-03440-y

Image Credits: AI Generated

DOI: 10.1038/s41567-026-03440-y

Keywords: altermagnetism, CrPS4, van der Waals heterostructures, twisted bilayers, spintronics, magneto-optical spectroscopy, Zeeman splitting, Raman spectroscopy, 2D magnets, spin splitting, antiferromagnetism, first-principles calculations