Scientists have unveiled a computational design for a terahertz metasurface biosensor that combines five distinct materials—graphene, MXene, tungsten disulfide, gold, and copper—into a single multilayer architecture capable of detecting refractive-index changes associated with chikungunya virus infection. The study, published in Results in Optics by G. Arunachalam, A.R. Kalaiarasi, Giri.G. Hallur, and V. Parthasarathy, reports a numerically predicted sensitivity of 907 GHz per refractive-index unit, among the highest values reported for hybrid-material terahertz biosensors, and couples the electromagnetic design to an XGBoost machine-learning surrogate model that predicts sensor behavior with accuracies exceeding 99.8 percent.

Chikungunya virus is an enveloped, positive-sense single-stranded RNA alphavirus transmitted primarily by Aedes aegypti and Aedes albopictus mosquitoes. While mortality remains low, generally below 0.1 percent, the broader public health burden arises from severe polyarthralgia that can persist for months or even years in a substantial fraction of patients across tropical and subtropical regions. With no approved antiviral therapy or licensed vaccine currently available, diagnostic accuracy sits at the center of clinical management and transmission control. Yet existing diagnostic tools face significant practical constraints, particularly in point-of-care and field settings.

Serological methods such as enzyme-linked immunosorbent assay suffer from cross-reactivity with related alphaviruses, including Ross River and O’nyong-nyong viruses, reducing diagnostic specificity. Real-time reverse transcription polymerase chain reaction offers high sensitivity during the acute phase but depends on laboratory infrastructure, trained personnel, and controlled sample handling. Virus isolation in cell culture provides confirmatory results but requires biosafety level-3 facilities and extended processing time. These limitations have motivated the search for rapid, label-free sensing approaches capable of operating at clinically relevant concentrations in decentralized environments.

The new work turns to the terahertz band, spanning roughly 0.1 to 10 THz, which lies between microwave and infrared regions of the electromagnetic spectrum. Terahertz radiation is non-ionising, with photon energies in the millielectronvolt range, allowing interaction with biological samples without causing ionising damage. Crucially, terahertz waves couple to low-frequency vibrational and rotational modes of biomolecules such as proteins, nucleic acids, and polysaccharides, producing measurable spectral features that support label-free detection. Because biological media including blood plasma and viral suspensions exhibit refractive-index variations in this domain, differences on the order of 0.01 to 0.02 refractive-index units can shift resonance frequency and field distribution when coupled to resonant sensing structures.

The proposed sensor is built around a periodically repeated square unit cell containing two identical square resonators, 2000 nanometers on a side, coated with tungsten disulfide—a two-dimensional transition metal dichalcogenide with strong light–matter interaction, high carrier mobility, and excellent dielectric confinement. This layer localizes the electric field in the sensing region and increases the interaction volume between the resonant field and the surrounding analyte. A pair of concentric circular rings, 3200 and 3800 nanometers in diameter and functionalized with MXene, surrounds the resonators, enhancing electromagnetic coupling, promoting efficient charge transport, and strengthening plasmonic interactions between resonator elements. An outer gold ring, 6000 nanometers in diameter, serves as the main plasmonic resonator supporting strong localized surface plasmon resonances with low optical loss. The entire multi-resonant assembly is embedded within a monolayer graphene sheet measuring 14,000 by 14,000 nanometers, which acts as a broadband absorber and a high-mobility charge transport medium whose electrically tunable chemical potential allows dynamic, active control of the resonance characteristics. Copper elements woven into the architecture provide supplementary conductive pathways that reinforce current distribution and coupling among the hybrid resonators. The structure rests on a silicon dioxide substrate, chosen for its low dielectric loss and mechanical robustness, which preserves the resonance quality factor.

The authors stress that the multilayer design is compatible with established microfabrication techniques. Fabrication would begin with a cleaned silicon dioxide substrate, followed by polymer-assisted transfer of monolayer graphene and thermal annealing. Gold and copper resonators can be deposited through electron-beam evaporation or magnetron sputtering with lift-off patterning, MXene rings through solution-based deposition, and the tungsten disulfide layer through chemical vapor deposition or thin-film transfer. With a unit-cell dimension of approximately 14 micrometers, the required feature sizes fall comfortably within conventional photolithography capabilities, and because fabrication imperfections remain far smaller than the operating terahertz wavelength, moderate tolerances are expected to have only limited influence on the electromagnetic response. For biosensing, the graphene surface could subsequently be functionalized with chikungunya-specific antibodies or aptamers using EDC/NHS or pyrene-based linker chemistries.

The electromagnetic modeling, performed in COMSOL Multiphysics, reveals a pronounced resonance at 0.355 THz, where electric-field simulations show the strongest and most spatially extensive enhancement. Varying the graphene chemical potential from 0.1 to 0.9 electronvolts produced dramatic modulation of the transmission contrast, which increased roughly 36-fold at the highest bias, from less than one percentage point to more than 27 percentage points, while peak transmission remained essentially unchanged above 98.6 percent. This asymmetric response—stable maxima paired with strongly modulated minima—arises because increasing chemical potential enhances graphene’s intraband conductivity, deepening resonant absorption through stronger light–matter interaction. The sensor also demonstrated remarkable angular robustness, maintaining peak transmittance above 97.8 percent across incident angles from 0 to 80 degrees, an important tolerance for realistic deployment where normal incidence cannot always be guaranteed.

Geometric optimization identified an outer circular resonator radius of 2500 nanometers and a square resonator side length of 2500 nanometers as the dimensions that maximize resonance strength, driving minimum transmittance down to approximately 65 to 67 percent and signifying optimal electromagnetic confinement within the active sensing region. When evaluated across refractive indices from 1.33 to 1.39, a range encompassing reported values for healthy plasma (approximately 1.35 RIU), healthy platelets (1.39 RIU), chikungunya-infected plasma (1.33 RIU), and infected platelets (1.38 RIU), the optimized configuration achieved a maximum sensitivity of 907 GHz/RIU, a figure of merit of 18.519 RIU⁻¹, a minimum detection limit of 0.061 RIU, a sensor resolution of 55.178, and a refractive-index resolution of 0.001 RIU. In comparative benchmarks against recently reported graphene- and hybrid-material-based terahertz biosensors—including graphene/copper/aluminum, graphene/silicon dioxide, MXene–graphene–black phosphorus, and graphene-integrated copper–silver designs—the proposed architecture’s sensitivity exceeded most published values, reflecting the synergistic field confinement produced by the five-material stack.

The machine-learning component addresses one of the central bottlenecks in metasurface development: the computational cost of repeated full-wave finite-element simulations during structural optimization. The team trained an XGBoost regressor, an ensemble method based on gradient-boosted decision trees, on simulation data covering incidence-angle variation and circular-ring dimension variation, using stratified sampling with 80 percent of data for training and 20 percent held out for independent testing, alongside five-fold cross-validation and grid-search hyperparameter optimization. The model captured more than 99.7 percent of response variance on unseen data, with mean absolute percentage errors below 0.162 percent and prediction accuracies from 99.838 to 99.959 percent. Acting as a computational surrogate, the trained model enables rapid interpolation across the design space without sacrificing the fidelity of the underlying electromagnetic model.

The authors are careful to frame the work as a computational proof-of-concept rather than an experimentally validated diagnostic. The sensor detects changes in the local dielectric environment surrounding the resonators, not viral particles or nucleic acids directly, and refractive-index sensing alone cannot intrinsically distinguish chikungunya from other viruses with similar dielectric properties. Analytical specificity in practice will depend on surface biofunctionalization with chikungunya-specific recognition molecules immobilized on the graphene or gold sensing surface. Future work will focus on device fabrication, biofunctionalization, calibration with reference refractive-index standards, experimental validation with clinically relevant plasma and platelet samples, and systematic evaluation of selectivity against related arboviruses—steps that would transform this numerically validated platform into a practical tool for rapid, label-free infectious disease diagnostics in the field.

A single gram of crushed oyster shell pulled in up to around 1.5 grams of rare earth metal, more than the shell’s own weight, out of water spiked with the kind of contamination you would expect from severe industrial pollution. A throwaway material, the sort of thing you might crunch underfoot on a beach, holding more than its mass in metals that the electronics and clean-energy industries fight over.

That’s the finding a Trinity College Dublin team reported in early 2026. Below, we walk through what they actually did, why oysters won and mussels and cockles fell short, and how much of this holds up once you leave the lab bench.

What the researchers actually did

The setup was low-tech at the front end. As reported in The University Times, the team collected oyster, mussel and cockle shells from beaches around Dublin, cleaned them, dried them, and crushed them into small grains. Then they dropped those grains into solutions carrying three rare earth elements and watched what happened over time. The three metals were lanthanum, neodymium and dysprosium, at concentrations chosen to mimic badly polluted water.

The chemistry underneath is where it gets interesting. Shells are mostly calcium carbonate, and in the rare-earth-rich water that calcium carbonate dissolves and is replaced, more or less on the spot, by new solid minerals built around the rare earth metals. In the peer-reviewed paper, published in Science of the Total Environment, the crushed shells were reacted with the metal-rich solutions across a wide range of temperatures, 25 to 205 degrees Celsius for up to three months. What you end up with is not the original shell soaking up metal like a sponge, but the shell being rebuilt, mineral by mineral, into something new.

As principal investigator Juan Diego Rodriguez-Blanco put it, “the process is entirely mineral-driven – the shells naturally transform dissolved rare earth elements into new solid minerals, so this isn’t a process that is difficult to drive, or one that requires much financial outlay or technical equipment.” That last part is a lab-based expectation rather than a proven cost sheet, but the appeal is easy to see: no exotic reagents, no elaborate rig, just waste shell and time.

Why oysters won and the others didn’t

All three shells are made of the same basic stuff, so why did oysters run away with it? The answer is structure. Oyster shell has a porous, layered microstructure, and that lets the dissolve-and-replace reaction work its way right through the grain rather than stalling at the surface. That’s how oyster grains reached uptake of up to roughly 1.5 grams per gram.

Mussels and cockles behaved differently, and the reason is more physical than chemical. A crust of new mineral formed quickly on the outside and then sealed the grain off, so the reaction couldn’t reach the inside. In those shells, the study reports that more than half of the original shell was left unchanged once that crust became impermeable. Same starting material, very different outcome, decided by how the shell is put together.

Why pull rare earths out of water at all

Rare earth elements are the unglamorous ingredients in a lot of modern hardware, from the magnets in wind turbines and electric motors to phone components. Mining them is expensive and messy, and industrial processing can leave behind runoff carrying dissolved metals. Two problems are tangled together here: recovering a valuable resource, and cleaning up water that carries it.

The shell angle is attractive partly because the raw material is a nuisance to begin with. As first author Rémi Rateau described it, “every year, the global aquaculture industry generates millions of tonnes of shell waste, much of which is discarded or sent to landfill.”  Rateau also noted that “relatively small amounts of shell waste could remove substantial quantities of rare earth metals from contaminated water.” 

What this is, and what it isn’t

This is one study, and it is a lab result. The concentrations were high by design, the conditions were controlled, and real industrial wastewater is a far messier soup of competing ions, organics and pH swings than a prepared solution in a flask. The team is upfront that the work still needs pilot-scale testing under genuine conditions before anyone talks about a deployable cleanup system. A promising lead is not a finished product, and this sits firmly in the former category.

Our read lands roughly where the team’s does. The measured uptake is striking, the input material is cheap and otherwise wasted, and the driving chemistry looks simple. Rodriguez-Blanco frames the ambition plainly: “we can start designing” low-cost strategies to remove critical metals from contaminated waters while giving new value to a major waste product. Start designing is the honest verb. On the strength of these numbers, it’s a lead worth taking out of the beaker and testing against the real, dirty water it was built to handle.

The post Researchers picked oyster, mussel and cockle shells off Irish beaches, crushed the 3 kinds of shell into grains, and dropped them into water carrying rare earth metals at concentrations similar to badly polluted industrial runoff. The oyster grains took up as much as roughly 1.5 grams of metal for every gram of shell. appeared first on Space Daily.

Attractive interactions between particles do not necessarily lead to these particles aggregating. This is the new finding from researchers at the Tokyo University of Science in Japan who say this behaviour comes from non-reciprocal interactions amongst particles of different sizes that exert unequal forces on each other. These interactions could serve as a general design principle for creating active matter with a structure that continuously reorganizes rather than simply relaxing towards a static state.

Nature abounds with examples of active matter, with some well-known examples being flocks of birds, fish and insects, sheets of biological cells and swarms of bacteria. Researchers have been trying to copy this collective behaviour and make synthetic active materials – such as self-propelled colloids and dense phases of mechanically driven biopolymers – in the laboratory. Before such applications see the light of day, however, they need to first understand the fundamental physics of these systems.

In their new study, a team led by Yutaka Sumino and Kiwamu Yoshii of Tokyo University of Science’s department of applied physics, came across a system in which colloidal particles interact through electrohydrodynamic flow. “We found that mixing particles of two different sizes in such a system completely changed the collective behaviour of these particles,” explains Sumino. “Instead of forming increasingly large crystal-like aggregates, as is the case for particles with the same size, the system remained highly dynamic.”

The physicists’ experiment involved suspending polystyrene colloidal particles with radii of 1 and 1.5 µm in water and confining them between transparent indium tin oxide-coated electrodes. This set up allowed them track more than 10,000 particles and quantitatively analyse their dynamics for more than an hour, as opposed to just a few minutes in previous such studies.

Newton’s third law holds

When the researchers applied an alternating electric field to the system, they observed electrohydrodynamic flows around the particles, the strength of which increased strongly as the size of the particles became bigger. The electrohydrodynamic-mediated attractive interactions therefore became asymmetric, they explain, something that causes larger particles to attract smaller ones more strongly. This imbalance leads to non-reciprocal interactions, meaning that a larger particle pushes a smaller one, but a smaller one doesn’t push back.

While the effective interaction between the particles appears to break Newton’s third law, it doesn’t because there is no violation of momentum conservation, explains Sumino: momentum is transferred to the surrounding fluid through the induced flows and is ultimately dissipated through friction with the substrate.

The researchers also observed that particles of different sizes spontaneously pair together to form asymmetric structures with a distinct front and tail. These pairs behave as self-propelled units and move through the suspension – even though individual particles cannot propel themselves. As more self-propelled pairs form, they assemble into larger clusters, but these clusters do not continue growing into large aggregates and instead repeatedly fragment, rearrange themselves and reform. Importantly, they note: “the larger particle tends to be at the front of these moving pairs. This head-heavy size asymmetry, together with excluded-volume interactions, promotes the fragmentation of larger clusters and thereby prevents continuous coarsening”.

Sumino and Yoshii combined their experiments with numerical simulations, thereby identifying non-reciprocal pair motion as the microscopic origin of this behaviour.

The results, they say, suggest that non-reciprocal interactions can serve as a general design principle for creating active materials with structures that continuously reorganize rather than simply relaxing toward a static state. “An interesting aspect,” explains Sumino, “is that self-propulsion does not need to be built into each individual particle: it can emerge collectively from non-reciprocal interactions between particles that do not self-propel on their own.”

If such interactions can be controlled externally, they could provide a way to design microscopic systems that collectively gather, transport, fragment or mix materials, he tells Physics World. “Possible directions include programmable active materials and microrobotic systems, although these applications are still some way off.”

Reporting their findings in Physical Review Letters, the researchers say they would now like to understand how general this non-reciprocal mechanism is and whether the same principle – collective activity emerging from non-reciprocal interactions – can be transferred to other experimental systems. In the system studied in this work, the strength of the non-reciprocity depends on parameters such as particle size, composition and the applied electric field. By systematically tuning these parameters, they aim to explore what other types of collective states can be generated and whether transitions between them can be controlled.

“A more detailed hydrodynamic analysis of the system is also needed to develop a quantitative understanding of the interactions and ultimately predict the collective motion of the colloids,” says Sumino.

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Concrete is the second most-used substance on Earth after water, and the cement that holds it together is one of the biggest headaches in climate policy.

Making cement means heating limestone until it breaks down, a process that accounts for around 8 percent of global CO2 emissions. And we are not slowing down. Concrete production is projected to climb from roughly 14 billion cubic metres a year toward 20 billion by mid-century. So anything that lets you use less cement per wall, without the wall getting weaker, is worth a close look.

That is the claim in a new paper out of India, and it comes with an unusual ingredient: human waste. A team turned treated sewage sludge into a fine black powder, swapped it in for a tenth of the cement, and got concrete that ended up harder to crush, not softer.

It’s one study, and an early-access version still subject to edits, so treat what follows as a promising result, not a settled fact.

Why cement is the part worth fixing

The problem with concrete is not the gravel or the sand or the water. It is the cement paste that binds them together. Estimates of cement’s climate footprint vary a bit depending on how you count, landing somewhere between 5 and 8 percent of global human-caused CO2 emissions each year. Most of that comes from the chemistry of making cement, not the fuel in the furnace. That is why switching to cleaner energy alone will not fix it.

One line of research has spent years asking whether you can replace some of that cement with biochar. Biochar is a carbon-rich char made by heating organic matter with very little oxygen. The appeal is twofold: you use less cement, and the carbon in the char, which the original plant or waste pulled out of the air, gets locked inside the building instead of released. The open question has always been whether the concrete survives the swap.

What the Warangal team actually did

The material here started at a sewage-sludge treatment plant in Warangal, in the Indian state of Telangana, a facility set up as a public-private partnership to tackle the city’s sewage problem. Human solid waste that would otherwise be dumped or burned gets processed there.

V Srinivas Chary, whose institution is a partner on the project, praised the operation, saying that “From the toilet to the final conversion of the biochar, the process is scientific and comprehensive.” 

The research team, led by civil engineer Raghuvesh Tiwari of Manipal University Jaipur with colleagues at Louisiana Tech University, took that sludge, dried it, and heated it to between 350 and 450 degrees Celsius with very little oxygen. That heating step drives off gases and leaves char behind. They ground the char into a powder, then replaced 5, 10, and 15 percent of the cement in test concrete with it and ran the usual checks: how well it resisted crushing and bending, how much it shrank, how much water it soaked up, how many gaps it had inside.

Dumped waste versus a carbon sink in the wall

The study sets up a pointed contrast. Normally a city’s sewage sludge is a liability to be disposed of, and a builder’s cement is a material whose production loads the atmosphere. Both are problems flowing toward waste and emissions. The first is bigger than you might think: a single person produces up to around 400 grams of faeces a day, which across a city becomes a constant stream that has to go somewhere.

The idea here is to route that stream into the wall. The waste replaces some of the cement, and the carbon in the char stays put instead of being burned off. It is a tidy story on paper: two problems that might cancel out a slice of each other. The caveat is that this study did not actually measure the carbon side of that ledger, so the climate benefit is still an argument rather than a number the paper delivered.

Why it got stronger, not weaker

The surprise is that adding a char to concrete made it tougher. The study reports this clearly for the middle dose. As the authors put it, “After a 91-day curing period, concrete with a 10% biochar substitution demonstrated a 21% increase in compressive strength and a 42% increase in flexural strength.” The 5 percent mix was close behind, gaining about 20 percent in crushing strength and 36 percent in bending strength over the same period. Those gains are measured against this study’s own plain concrete, not a universal benchmark.

There is a limit, though, and the study found it. The 15 percent mix did worse. Past a certain point, the char stops helping and simply dilutes the cement, a useful reminder that the good result here depends on getting the dose right. The authors’ own summary is careful: the results indicate that biochar at the 5 and 10 percent levels “yields significant improvements in concrete properties.”

The catch, and what still has to be proven

A strong lab result is not a paving slab. Several things stand between this and a real building site, and the paper is upfront about the biggest one. Sewage sludge can carry heavy metals, and whether those slowly leak out of the finished concrete over time is an open question. That is not something you can wave away when the material might end up in a school foundation or a water channel.

How long it lasts is the other unknown. The tests ran to 91 days in controlled conditions. Real structures face decades of freezing and thawing, wetting and drying, weight and vibration, and nobody has shown yet how this concrete holds up over that span. The researchers say themselves that more work is needed to assess how it performs under real-world conditions such as freeze-thaw cycles and extreme temperatures. 

What the study earns, we think, is attention rather than adoption. The explanation is plausible, the numbers are real for the mix that was tested, and the idea that a waste stream and a carbon-heavy material might each solve part of the other’s problem is worth chasing. 

The post Researchers took biochar made from human waste at a treatment plant in Warangal, India, ground it into a fine black powder and mixed it into concrete in place of a tenth of the cement. After 91 days the concrete was 21 percent harder to crush. appeared first on Space Daily.

Scientists led by the Universitat Politècnica de Barcelona and the ESRF, the European Synchrotron, have revealed the chemical reactions in nanoparticles that created a unique, shimmering effect in the painting on ninth-century Islamic ceramics. The results are published in Science Advances.

Scientists led by the Universitat Politècnica de Barcelona and the ESRF, the European Synchrotron, have revealed the chemical reactions in nanoparticles that created a unique, shimmering effect in the painting on ninth-century Islamic ceramics. The results are published in Science Advances.

A yeast-based material turns Mars’s hostile environment into part of the construction process.

My guest in this episode of the Physics World Weekly podcast is Marco Caffio, who is co-founder of iGii. The UK-based company has developed a porous 3D carbon nanomaterial called Gii, which it manufactures for a range of customers and applications.

Caffio describes Gii as a 3D version of graphene that delivers many of the useful electronic, structural and chemical properties of that 2D wonder material. Unlike graphene, which can be difficult to manufacture and integrate into devices, Caffio explains that Gii can be customized for use in practical technologies such as chemical sensors, batteries and heaters.

We also chat about the genesis of the company and Caffio offers career advice to physicists who are keen on working in the materials-technology sector.

The post Porous material offers the benefits of graphene in 3D appeared first on Physics World.

Nickel disulfide, NiS₂, is an unusual material. Its interior behaves as an electrical insulator, blocking the flow of current, while certain parts of its surface conduct electricity freely. This contradiction has puzzled physicists for decades, and a recent study offers a detailed explanation. The conduction, it turns out, is confined to tiny one-dimensional steps on the surface, and these narrow conducting channels are a direct consequence of how electrons are arranged inside the crystal.

Mott insulator

NiS₂ shares its crystal structure with pyrite, the mineral sometimes called fool’s gold. At ordinary conditions, its electrons do not move freely through the lattice: the strong electrical repulsion between them opens an energy gap that keeps the bulk of the material insulating. This behaviour defines what physicists call a Mott, or charge-transfer, insulator, a category of materials that simple band theory cannot explain. Band theory predicts that any material with a partially filled outer electron shell should conduct electricity, yet compounds like nickel oxide obstinately insulate instead. In 1949, the physicist Nevill Mott showed why: if two electrons crowding the same atomic site cost more energy than is available, the electrons stay locked in place and no current flows, regardless of what band theory predicts. NiS₂ behaves in much the same way.

Yet electrical-transport measurements had repeatedly found that crystals of NiS₂ conduct through their surfaces. Earlier scanning-tunnelling experiments helped explain part of the puzzle: the broad, flat surface remains insulating, but the electronic gap narrows sharply near atomic-scale steps, tiny cliffs just one or two atoms high where the surface abruptly changes level. A new study ¹ confirms this picture and examines the steps with much higher spatial resolution than before.

A not that smooth surface

A scanning tunnelling microscope (STM) works by bringing an extremely sharp needle close enough to a surface that electrons can leak across the gap between tip and sample. Measuring that leakage at different energies reveals both the surface’s shape and how easily electrons can enter or leave it. In NiS₂, these measurements uncovered electronic states concentrated tightly along the step edges: the states extend for a long distance parallel to a step but stay confined to roughly one to two nanometres across it. In other words, the electrons are effectively free to move in one direction while remaining trapped in the other two, forming genuine one-dimensional conduction channels.

Two distinct kinds of surface step were observed, depending on whether the outermost atomic layer exposes nickel or sulfur atoms. Both carry electronic states associated with their steps, but they are not equally important. The nickel-terminated steps host states that sit right at the energy where electrical conduction happens, making them the most likely explanation for the surface conductivity that transport experiments had measured for years. The sulfur-terminated steps also host states inside the insulating gap, but their energy is positioned further away, making them less directly responsible for carrying current.

The study also tested what happens when a magnetic field is applied perpendicular to the surface. The step-edge states survived fields as strong as 10 tesla, a field far stronger than any produced by a laboratory magnet used for everyday purposes. Their intensity weakened somewhat, particularly on the sulfur-terminated steps, but the states were never destroyed. This robustness matters because NiS₂ already displays complicated magnetic behaviour of its own, which might easily have disrupted a more fragile conducting state.

The deeper explanation lies in how electrons are distributed inside the crystal. In an ordinary insulator, the occupied electronic states can usually be pictured as clouds centred on the atoms themselves. NiS₂ is different: calculations show that part of its electron density is naturally centred at special positions between atoms, specifically near the midpoints of pairs of sulfur atoms, where no atom actually sits. Physicists call these obstructed charge centres, and materials built around them belong to a class known as obstructed atomic insulators, a concept that grew out of a broader effort over the past decade to classify materials by the geometry, and not just the energy, of their electron states.

Not ordinary metallic defects

This arrangement is not a chemical accident; it is protected by the symmetry of the crystal. When the crystal is cut to form a surface, though, the special positions that hosted this electron density can no longer fit together the way they did in the bulk. At a step edge, this mismatch creates what is known as a filling anomaly: an imbalance in how the available electronic states can be occupied, which leaves behind a state inside the insulating gap, localized right at the edge. Calculations of artificial step structures reproduce the main features seen under the microscope, including this localization, supporting the idea that the conducting channels are not ordinary metallic defects caused by broken chemical bonds, but a direct consequence of the crystal’s underlying electronic geometry.

The study also settles a long-standing question about NiS₂’s magnetism at low temperature. Below about 39 kelvin, the nickel magnetic moments arrange themselves into a non-collinear antiferromagnetic pattern, with neighbouring moments pointing in different directions that largely cancel out. Below about 30 kelvin, they rearrange again into a state that produces a small but measurable net magnetization. New neutron-diffraction measurements support a revised model of this low-temperature arrangement, resolving a debate about the material’s magnetic ground state that had lasted for roughly fifty years.

Taken together, the results give NiS₂ a significance well beyond its peculiar conducting surface. The material shows that strong repulsion between electrons, magnetism, and the geometry of electronic states can coexist within a single crystal. Such geometric protection is normally associated with topological materials, in which certain electronic states are shielded from disorder or defects by the crystal’s symmetry. The new results suggest that this protection can survive even when strong interactions push a material into being an insulator, making NiS₂ a rare setting for studying how the quantum-mechanical arrangement of electrons decides where electricity is allowed to flow within an otherwise insulating crystal.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.

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