The central obstacle the researchers had to overcome was a practical one that has haunted the nickelate field since its inception. Neutron scattering is the gold-standard technique for measuring magnetic order and spin excitations in solids, because neutrons carry magnetic moments and scatter off the spins in a crystal in a way that reveals their spatial arrangement and their energy spectrum. But neutrons interact weakly with matter, so the technique demands large, high-quality single crystals, often measured in batches. Growing such crystals of La3Ni2O7 proved extraordinarily difficult, and progress on the magnetic ground state of this pivotal compound had been correspondingly slow. By assembling a mosaic of single crystals large enough for spectroscopy, the team cleared the single biggest experimental bottleneck in the field and opened the door to a definitive measurement.

What they found in the static magnetic order was a single-stripe pattern. In stripe order, spins arrange themselves into periodic antiferromagnetic bands separated by domain walls, a configuration familiar from certain layered nickel oxides and from doped cuprates, where stripes have long been suspected of playing a role in pairing. The observation of single-stripe order in the parent compound of a high-temperature nickelate superconductor is significant because it tells theorists what magnetic texture the electrons naturally want to form before superconductivity is induced by pressure. The order also coexists with a finite spin gap, meaning there is an energy threshold below which magnetic fluctuations are suppressed, a feature that shapes how the material can respond when superconductivity sets in.

The dynamical measurements revealed an equally rich picture. The spin excitations of La3Ni2O7 are anisotropic within the plane, meaning their energies and intensities depend on the direction of travel through the crystal lattice, a direct fingerprint of the stripe arrangement. More strikingly, the excitations show pronounced modulation along the direction perpendicular to the layers, revealing that the two nickel oxide planes in each bilayer are antiferromagnetically coupled to one another. This interlayer coupling is not a minor detail. In the bilayer structure of La3Ni2O7, the coupling between the paired planes is widely believed to be central to the superconducting mechanism under pressure, with several theoretical works proposing that interlayer exchange drives the pairing interaction. The neutron data now provide direct experimental grounding for that picture in the magnetic sector.

To make quantitative sense of the spectra, the team compared their measurements against a bilayer Heisenberg-type model, the standard theoretical framework for describing localized spins coupled within and between layers. The model captures the measured dispersion, the relationship between excitation energy and momentum, provided it includes strong interlayer exchange and competing in-plane couplings. That the data are well described by such a model does not mean the material is a simple insulator of localized moments; rather, it gives theorists a concrete set of exchange parameters extracted from experiment, replacing the guesswork that has plagued model calculations of the nickelate phase diagram. The competing in-plane couplings in particular hint at the delicate energy balance that pressure must tip to produce superconductivity.

Perhaps the most consequential result concerns the absolute intensity of the magnetic fluctuations, not merely their energies. By carefully normalizing the measured scattering cross sections, the researchers could compare the strength of spin fluctuations in La3Ni2O7 directly with those in cuprate superconductors. The spin-wave bandwidth, the total energy range over which well-defined spin waves propagate, turned out to be only about twenty-five percent of the corresponding bandwidth in the cuprates. On its face, that might suggest the nickelate is a much weaker magnet. But the local dynamic susceptibility, a measure of the fluctuating moment summed over momentum space, tells a different story: at comparable energies, the susceptibility in the nickelate is enhanced relative to the cuprates, so that the total fluctuating moment is comparable between the two families.

This combination, a narrow bandwidth but intense mid-energy fluctuations, points to substantial electronic correlations in the nickelate and to a distribution of magnetic spectral weight that differs fundamentally from the cuprate case. In the cuprates, spin excitations extend to very high energies, several hundred millielectronvolts, reflecting the large superexchange interaction between copper spins. In La3Ni2O7, the spectral weight is concentrated at intermediate energies, which matters because theories of unconventional superconductivity generally hold that spin fluctuations in a particular energy window provide the glue that binds electrons into Cooper pairs. A material whose fluctuations are intense precisely in the mid-energy range may pair electrons through a mechanism that is qualitatively related to, but quantitatively distinct from, the cuprate mechanism.

The findings arrive at a moment of extraordinary ferment in the nickelate field. Since the initial report of superconductivity near 80 kelvin in pressurized La3Ni2O7 in 2023, and the subsequent demonstrations of ambient-pressure superconductivity in thin films and of bulk superconductivity up to 96 kelvin in pressurized nickelate single crystals, researchers have been racing to determine whether nickelates constitute a genuinely new route to high-temperature superconductivity or a variation on the cuprate theme. The magnetic measurements now reported sharpen that debate considerably. The single-stripe order, the strong bilayer coupling, and the mid-energy-weighted fluctuation spectrum together constitute a magnetic framework that is distinct from the cuprates, even as the presence of strong correlations and antiferromagnetism echoes the older family.

For the immediate future, the study sets a benchmark. Any credible theory of superconductivity in La3Ni2O7 must now reproduce a spin excitation spectrum with these specific features: a finite spin gap, anisotropic in-plane dispersions governed by competing exchanges, bilayer modulation from antiferromagnetic interlayer coupling, and an absolute fluctuating moment comparable to that of the cuprates despite a much reduced bandwidth. The work also demonstrates a methodology, mosaic neutron spectroscopy on carefully grown crystals, that can now be extended to chemically substituted and pressurized variants, tracing how the magnetic framework evolves as superconductivity emerges. In a field where the pairing mechanism remains unsettled, knowing precisely what the parent compound’s spins are doing is the essential first step toward understanding what happens when those spins conspire to superconduct.

Subject of Research: Magnetic order and spin excitations in the bilayer nickelate La3Ni2O7, the parent compound of a high-temperature nickelate superconductor

Article Title: Single-stripe magnetic order and bilayer spin dynamics in single-crystalline La3Ni2O7

Article References: Chen, L., Zhang, E., Hao, Y., Zhu, Y., Cui, B., Abernathy, D. L., Williams, T. J., Ikeda, Y., Zhang, H., Liu, F., Wang, W., Wang, Q., & Zhao, J. (2026). Single-stripe magnetic order and bilayer spin dynamics in single-crystalline La3Ni2O7. Nature Materials. https://doi.org/10.1038/s41563-026-02768-3

Image Credits: AI Generated

DOI: 10.1038/s41563-026-02768-3

Keywords: nickelate superconductors, La3Ni2O7, neutron scattering, stripe magnetic order, spin excitations, bilayer coupling, spin gap, high-temperature superconductivity, strong correlations, Heisenberg model, dynamic susceptibility, magnetism

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

Magic-angle twisted bilayer graphene has captivated the scientific community ever since its superconducting properties were first revealed. The material is fabricated by taking two atomically thin sheets of carbon atoms arranged in a honeycomb lattice and stacking them with a precise rotational twist of approximately 1.1 degrees. At this seemingly arbitrary angle, the electronic bands of the two layers flatten dramatically, electrons slow to a crawl, and interactions between them become overwhelmingly important. The result is a system that superconducts at temperatures only a few kelvin above absolute zero, yet behaves in ways that evoke the great unsolved mysteries of high-temperature superconductivity in cuprates and other exotic materials.

For nearly a decade, however, theorists have been split over what actually glues the electrons into the pairs that flow without resistance. Some propose that the pairing arises purely from collective electronic behavior — fluctuations and correlations among the electrons themselves. Others argue for a more mundane, conventional origin: phonons, the quantized vibrations of the atomic lattice, mediating the attractive interaction, much as they do in ordinary superconductors such as lead or niobium. Distinguishing between these scenarios experimentally has proven extraordinarily difficult, and a series of earlier screening experiments produced ambiguous, weaker results that left the debate simmering.

The Manchester-led team, which included collaborators from the Henry Royce Institute, Washington University in St Louis, the University of Pennsylvania, the University of Antwerp, Japan’s National Institute for Materials Science and the National University of Singapore, found a way to break the deadlock. Dr Julien Barrier, the lead author of the study, describes the two critical hurdles that had to be overcome. “To make a difference, we had to solve two issues,” he explained. “First, to build a device in which the screening layer sits extremely close, a fraction of a nanometre, to the superconducting graphene while remaining electronically separate. Second, we had to make that screening layer tuneable. To this effect, we used a twisted graphene bilayer in atomic contact to the magic-angle graphene.”

The device architecture is a masterpiece of nanoscale engineering. It consists of two twisted graphene bilayers separated by less than a nanometre — a gap so small that it approaches the scale of individual atomic bonds — yet the two systems remain electronically decoupled. This means electrons cannot hop between the layers, but the electric fields they generate can. By adjusting the carrier density in the adjacent screening layer, the researchers could continuously tune the strength of the Coulomb interaction — the repulsive electrostatic force between electrons — experienced by the superconducting layer. The closer proximity and the tunability of the screening layer allowed modifications of Coulomb interactions over distances as short as 0.3 nanometres, an unprecedented level of control in such experiments.

What happened next surprised even the team. “When we switched on the screening, we were surprised to find that superconductivity was completely suppressed,” said Professor Alexey Berdyugin of the National University of Singapore, a corresponding author of the study. “This provides clear experimental evidence that superconductivity in this system originates from strong electron-electron interactions. This behaviour offers a new opportunity to better understand the mechanisms underlying superconductivity in other materials with strong electronic interactions, including high-temperature superconductors.”

The results were unambiguous and strikingly dose-dependent. As the researchers progressively increased the carrier density in the neighbouring graphene bilayer, the superconductivity in the adjacent magic-angle layer weakened steadily. At sufficiently high carrier densities, superconductivity vanished entirely. Moreover, the superconducting critical temperature — the temperature below which the material loses all electrical resistance — could be reduced by more than an order of magnitude through screening alone. Alongside the suppression of superconductivity, the team observed that correlated insulating states, another enigmatic signature of magic-angle graphene in which the interacting electrons lock themselves into an insulating arrangement, disappeared under the same conditions. The parallel fates of superconductivity and the correlated insulating states reinforce the picture that both phenomena spring from the same source: strong electronic correlations.

The theoretical implications are profound. If phonons — lattice vibrations — were the glue binding electron pairs, screening the Coulomb interaction would be expected to leave superconductivity largely unchanged or even to enhance it slightly, since screening suppresses the repulsive Coulomb interaction that normally opposes conventional pairing. The Manchester team observed precisely the opposite: screening destroyed the superconducting state. According to the researchers, this inverted response rules out conventional phonon-mediated pairing as an explanation for superconductivity in this class of materials. The enhanced magnitude of the effect, far stronger than in earlier screening experiments, is attributed to the exceptionally small separation between the superconducting and screening layers, which allowed the Coulomb interaction to be modified far more effectively than ever before.

Importantly, the authors are careful about what the study does and does not claim. While the work does not identify a single definitive pairing mechanism, several unconventional theories remain consistent with the results, including those based on collective electronic interactions, such as fluctuation-driven pairing channels. What the experiment does accomplish is to place much tighter constraints on any future theory that hopes to explain superconductivity in magic-angle graphene. Any candidate mechanism must now survive a brutal test: it must predict that superconductivity collapses when Coulomb interactions are screened at the nanometre scale. Theories relying primarily on conventional phonon glue no longer pass.

Professor Sir Andre Geim, the Nobel laureate who first isolated graphene and a corresponding author of the new work, frames the achievement in the context of physics’ grandest prize: superconductivity at room temperature. “Personally, I am interested only in high-temperature superconductivity — preferably at room temperature or above,” he said. “This study was done at temperatures so low that even helium turns liquid. But unless we understand what makes superconductivity work, we are unlikely ever to reach room-temperature superconductivity, let alone make this remarkable phenomenon commercially useful. Our study takes only a tiny step — but still a step — in that direction, helping to nail down the mechanism of exotic superconductivity in graphene. Rome was not built in a day.”

Geim’s caution is well-founded. Room-temperature superconductivity remains the holy grail of materials science, promising lossless power transmission, ultra-efficient magnets and transformational electronics. But the path there runs directly through the problem that this study addresses: understanding the mechanism of pairing in strongly correlated electron systems. Because magic-angle graphene is tunable, clean and theoretically tractable in ways that cuprates are not, it has become a Rosetta Stone of sorts for the physics of unconventional superconductors. Each experimental advance in decoding it reverberates through the broader effort to understand and eventually engineer higher-temperature superconductors.

Beyond superconductivity itself, the technique developed by the team may have far-reaching applications. “In this study, we introduced a method for screening electron-electron interactions over scales as short as 0.3 nm, which turned out to be crucial for controlling superconductivity in magic-angle graphene,” Professor Berdyugin concluded. “We anticipate that this unprecedented level of short-range screening could also help clarify many other debated phenomena.” Strongly correlated materials are riddled with contested phenomena — strange metals, nematicity, competing orders, quantum criticality — and a controllable, short-range knob for dialling electron interactions up and down could prove decisive in disentangling them.

The study stands as a vivid demonstration of how atomically engineered van der Waals heterostructures — stacks of two-dimensional materials assembled layer by layer with atomic precision — can answer questions that bulk crystals cannot. By placing a tuneable electronic screen within a fraction of a nanometre of a superconducting sheet without contaminating it, the Manchester-led collaboration has effectively performed a controlled experiment on the fundamental interaction behind one of nature’s most subtle collective states. The superconductivity in magic-angle graphene, the evidence now says, belongs to the electrons themselves. The next chapter — identifying exactly how they conspire to pair — is still to be written, but the rules of the game have just been redrawn.

The result addresses one of the most persistent puzzles in condensed-matter physics. In ordinary metals, electrons occupy a broad range of available energy states, allowing them to move freely through a crystal. In a Mott insulator, however, strong repulsion between electrons blocks that motion even when conventional band theory predicts metallic behaviour. Adding carriers, a process known as doping, can eventually produce a metal and, in some real materials such as cuprate superconductors, superconductivity at remarkably high temperatures.

The simplest theoretical framework for this problem is the Hubbard model. It describes particles hopping between sites of a lattice while paying an energetic penalty whenever two particles occupy the same site. The competition between kinetic motion and on-site repulsion can generate insulating, metallic and potentially superconducting phases. Despite its apparent simplicity, the model is notoriously difficult to solve in the regime where interactions are strong and the system is only partially filled, precisely the conditions associated with the unusual metallic states of cuprates.

In the new study, researchers created a highly controlled version of the Hubbard model using a quantum simulator based on ultracold atoms. The atoms were arranged in an artificial lattice that mimics the crystal structure experienced by electrons in a solid. By tuning the interaction strength, density and temperature, the team could explore regions of the phase diagram that are difficult to access in conventional materials, where disorder, chemical complexity and competing effects can obscure the underlying physics.

The experiment benefited from a substantial reduction in achievable temperatures, allowing the atoms to enter a regime where subtle thermodynamic signatures become visible. One of the central measurements was the compressibility, which describes how strongly the density of a system changes when its chemical potential is varied. In practical terms, it reveals how easily particles can be added. The researchers found that, upon cooling, the compressibility develops a maximum at intermediate doping.

That maximum is more than a feature in a graph. It marks an inflection point in the equation of state, meaning that the relationship between particle density and chemical potential changes its curvature. By following the position of this maximum as the interaction strength was varied, the researchers identified a continuous line of thermodynamic anomalies. At strong interactions, this line separates two qualitatively different metallic regimes: an underdoped metal on one side and an overdoped metal on the other.

The team then used lattice modulation spectroscopy to investigate the system’s electronic response at different momenta. In this technique, the artificial lattice is periodically shaken, and the atoms’ response reveals how readily the system can absorb energy. The measurements showed a suppression of low-energy response in the underdoped regime. This loss was particularly pronounced near the antinodal regions of the Brillouin zone, the momentum-space areas associated with directions where the pseudogap is expected to be strongest.

A pseudogap is not a full insulating gap. Instead, it is a partial depletion of available low-energy electronic states. The distinction is crucial: a pseudogapped system may remain metallic, yet its charge carriers do not behave as they would in a conventional metal. The momentum-selective nature of the observed suppression provides an important clue that the phenomenon is linked to strong correlations and collective organization rather than simply to a uniform loss of particles or increased disorder.

By combining thermodynamic measurements with spectroscopic probes, the researchers constructed a pseudogap phase diagram for the Hubbard model as a function of interaction strength and doping. This gives experimental substance to a region of the theoretical phase diagram that has been discussed for decades but has been challenging to isolate in real materials. The observations suggest that the boundary between underdoped and overdoped behaviour is not merely a change in transport properties, but is connected to measurable changes in the system’s thermodynamics and excitation spectrum.

The findings do not yet identify the ultimate microscopic origin of the pseudogap, nor do they demonstrate high-temperature superconductivity in the simulated system. They do, however, establish a powerful platform for testing competing explanations, including possible connections between the pseudogap and charge order. Future experiments may examine whether density patterns, pairing correlations or other forms of hidden organization emerge in the same region. For now, the work shows that cold-atom quantum simulators can reproduce and dissect one of the most enigmatic states in correlated-electron physics, bringing researchers closer to understanding why doped Mott insulators can become both anomalous metals and, in related materials, superconductors.

Subject of Research: Doped Mott insulators, the Hubbard model and the pseudogap metal in strongly correlated quantum systems

Article Title: Pseudogap in a Fermi–Hubbard quantum simulator

Article References: Kendrick, L.H., Kale, A., Gang, Y. et al. “Pseudogap in a Fermi–Hubbard quantum simulator.” Nature (2026). https://doi.org/10.1038/s41586-026-10875-z

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41586-026-10875-z

Keywords: Fermi–Hubbard model, Mott insulator, pseudogap, quantum simulation, ultracold atoms, strongly correlated matter, anomalous metal, cuprate superconductors, compressibility, lattice modulation spectroscopy

The resurgence of interest in nickelate superconductors has been fueled by the discovery of La3Ni2O7 exhibiting superconductivity around 80 K above 14 GPa. However, the quest for higher transition temperatures (Tc) and the preparation of single crystals without resorting to complex high oxygen-pressure growth has hindered progress. The novel approach used in this study bypasses these challenges by synthesizing bilayer La2SmNi2O7−δ single crystals via flux growth at ambient pressure, achieving remarkable homogeneity and crystallinity evident from extensive characterization methods. These techniques include energy-dispersive spectroscopy (EDS), single-crystal X-ray diffraction (XRD), nuclear quadrupole resonance (NQR), and scanning transmission electron microscopy (STEM), all confirming the impeccable quality of the as-grown crystals.

The measured superconducting properties of La2SmNi2O7−δ single crystals are among the most impressive for nickelates to date. Resistivity measurements under pressure reveal zero resistance indicative of bulk superconductivity, with the onset temperature (Tc,max onset) reaching 92 K and zero-resistance temperature (Tc,max zero) at 73 K around 21 GPa. Magnetic susceptibility experiments further underscore bulk superconductivity with a clear Meissner effect emerging at 60 K when subjected to 20.6 GPa. These results firmly establish the viability of high-pressure induced superconductivity in bilayer nickelates with an even wider superconducting window than previously observed in related compounds.

One of the most compelling outcomes of this study is the detailed structural analysis under low temperatures and high pressure, which uncovers that superconductivity can be hosted in both monoclinic and tetragonal crystallographic phases. This contrasts earlier reports that associated superconductivity primarily with one specific lattice symmetry, suggesting a more nuanced relationship between crystal structure and superconducting pairing mechanisms in nickelates. Such revelations challenge existing theoretical frameworks and invite fresh perspectives on the interplay between lattice distortions, electronic correlations, and superconductivity.

Another landmark finding concerns the correlation between lattice distortion at ambient pressure and the superconducting transition temperature under high pressure. The researchers demonstrate that a larger in-plane lattice distortion, tunable through partial La/Sm substitution (manifested in La1.57Sm1.43Ni2O7−δ), pushes the onset Tc to an unprecedented 96 K at comparable pressures. This correlation suggests that lattice strain engineering at ambient conditions can effectively optimize superconducting properties when external pressure is applied, providing a strategic roadmap for designing nickelate superconductors with enhanced Tc.

The synthesis of high-quality single crystals without the reliance on elevated oxygen pressures is a pivotal advancement that addresses a major bottleneck in the field. Traditional approaches to nickelate superconductors have faced reproducibility issues and complications due to oxygen stoichiometry control under extreme conditions. The flux growth method adopted here not only simplifies sample preparation but also enables more precise control over chemical composition and structural homogeneity, which are critical for reliable experimental investigations and potential applications.

Nuclear quadrupole resonance and scanning transmission electron microscopy have played crucial roles in verifying the intrinsic nature of superconductivity and the high crystalline order of the samples. NQR offers microscopic insights into the local electronic environments, confirming bulk superconductivity without impurity-driven artifacts. STEM imaging further reveals atomic-scale uniformity, bolstering confidence in the structural integrity and correlation between crystal lattice parameters and superconducting behavior.

The implications of these findings extend beyond nickelates, contributing to the broader understanding of unconventional superconductivity in correlated oxides. The coexistence of superconductivity with different structural motifs in La2SmNi2O7−δ offers valuable clues for exploring pairing mechanisms, particularly the interplay of electron correlation, lattice distortions, and multi-orbital physics. These insights may inspire new theoretical models and computational studies that better capture the complexity of high-temperature superconductivity.

Moreover, the observed pressure-induced enhancement of Tc approaching 100 K nudges nickelates closer to the realm traditionally dominated by cuprate superconductors, revitalizing interest in nickel oxides as a fertile platform for superconductivity research. The ability to engineer higher transition temperatures via chemical substitution and lattice manipulation underlines the tunability of this material class and sets the stage for further experimental and technological breakthroughs.

Future research directions will likely focus on extending the approach demonstrated here to other rare-earth-substituted nickelates and achieving superconductivity at ambient pressure through chemical or epitaxial strain engineering. This study paves the way toward practical applications by reducing complexity in sample synthesis and improving material stability and homogeneity, prerequisites for device integration and scalability.

In summary, the discovery of bulk superconductivity reaching 96 K in ambient-pressure grown bilayer nickelate single crystals represents a significant leap forward. The combination of sophisticated synthesis, multi-faceted characterization, and comprehensive structural analysis delivers a holistic understanding of the factors governing superconductivity in this emerging family of compounds. These findings hold promise for not only advancing fundamental condensed matter physics but also propelling nickelate superconductors closer to real-world applications in energy-efficient technologies.

Subject of Research: Bilayer nickelate superconductors synthesized at ambient pressure exhibiting bulk superconductivity at temperatures up to 96 K under high pressure.

Article Title: Bulk superconductivity up to 96 K in pressurized nickelate single crystals.

Article References:
Li, F., Xing, Z., Peng, D. et al. Bulk superconductivity up to 96 K in pressurized nickelate single crystals. Nature (2025). https://doi.org/10.1038/s41586-025-09954-4

Image Credits: AI Generated

In the environmental context of advanced thin-film growth techniques, oxide molecular beam epitaxy (OMBE) and pulsed laser deposition (PLD) each exhibit unique merits that cater to various aspects of material synthesis. OMBE stands out for its exceptional control over elemental stoichiometry, enabling atomic-layer-by-layer growth of intricate oxide structures. It also excels when it comes to the deposition of materials with precise atomic arrangements. Nonetheless, its operational requirements present constraints; it typically functions in low-pressure conditions, a necessity that significantly limits its oxidation capabilities, greatly reducing its effectiveness for certain applications.

On the other hand, pulsed laser deposition is revered for its versatility, cost-effectiveness, and capability to achieve high growth rates. PLD is particularly appealing due to its ability to operate effectively under higher pressure conditions compared to OMBE. Unfortunately, this method experiences challenges related to intuitive control of stoichiometry, complicating the growth of complex, large-unit-cell metastable structures. As such, while both techniques boast distinct advantages, they are beset by limitations that necessitate the development of novel methodologies for oxide material synthesis.

Recently, an innovative technique known as Gigantic-Oxidative Atomic-Layer-by-Layer Epitaxy (GOALL-Epitaxy) has emerged from the Laboratory of Superconductivity Mechanism at the Department of Physics, Southern University of Science and Technology (SUSTech), alongside the Quantum Science Center of Guangdong-Hong Kong-Macao Greater Bay Area (QSC-GBA). This cutting-edge method aims to revolutionize the synthesis of oxide materials by enhancing oxidative power while maintaining atomic-level precision in the growth of intricately designed oxide structures. GOALL-Epitaxy strategically combines elements from both PLD and OMBE, effectively alleviating their respective shortcomings while harnessing their strengths, resulting in a remarkable increment of oxidative capability by three to four orders of magnitude.

The implementation of GOALL-Epitaxy has led to substantial advancements in the synthesis of various complex nickelates and cuprates. A particularly noteworthy achievement demonstrated by the research team includes the successful creation of an artificially designed nickelate structure featuring alternating single and double NiO₂ layers. This innovative configuration is postulated to serve as a foundational structure for high-temperature superconductivity, opening doors to new research directions and possibilities within this field. Remarkably, this achievement exemplifies not only the potential of GOALL-Epitaxy in materials discovery but also broadens the parameter space for the exploration of novel high-temperature superconductors and other strongly correlated electronic systems.

At its core, the principle of GOALL-Epitaxy revolves around an atomic-layer-by-layer deposition process carried out in a strongly oxidative environment. This intricate approach can be likened to building with legos, wherein various oxide layers are meticulously assembled on an atomically smooth substrate, enabling researchers to achieve the desired architectural design. The strong oxidative power is derived from liquefied, purified ozone, which serves as the oxidation source injected directly onto the substrate surface at a high concentration and flow rate through a specially designed nozzle. This innovative mechanism ensures that ozone reaches the substrate quickly and maintains an elevated oxidation intensity even under high-temperature conditions, which is crucial for the synthesis of complex oxide materials.

Further refining its capabilities, GOALL-Epitaxy employs high-energy laser pulses to ablate single-element oxide targets, which facilitates significantly higher growth pressures in comparison to OMBE. Remarkably, this process still enables atomic-layer precision akin to that achieved with OMBE, thereby establishing a new standard for thin-film growth techniques. Compared to the existing methods of PLD and OMBE, GOALL-Epitaxy presents more flexibility across varying temperature conditions. Under elevated temperatures, the enhanced oxidation promotes the thermodynamic stability of materials, ultimately leading to kinetic improvements that enhance crystalline quality.

Moreover, when operating under low-temperature conditions, the supplementary kinetic energy resulting from laser ablation contributes to achieving higher lattice quality. This breakthrough expands the possibilities for a wider array of material systems and artificial lattice structures. The comprehensive advantages presented by GOALL-Epitaxy align harmoniously with the pressing needs for innovative material design and exploration, offering scientists a powerful tool to surmount conventional limitations faced in the synthesis of intricate oxide materials.

Throughout its research, the team has thoroughly outlined the methodology and successes of GOALL-Epitaxy, culminating in the publication of their findings in the fourth issue of National Science Review, scheduled for 2025. Dr. Guangdi Zhou from SUSTech and Dr. Haoliang Huang from QSC-GBA serve as co-first authors, underscoring the collaborative effort that has driven this transformative research forward. This publication not only celebrates their milestones in material synthesis but also informs the broader scientific community of the vast potential that lies in this innovative approach to oxide growth.

The depth of research conducted stands as a testament to the team’s commitment to pushing boundaries in the field of materials science. GOALL-Epitaxy not only represents a methodological advancement but also hints at a future where the intricacies of strongly correlated electronic systems can be manipulated with unprecedented precision. The optimization of this technique may pave the way for revolutionary discoveries in high-temperature superconductors, sparking a wave of innovation and inquiry across materials science disciplines.

As researchers continue to delve into the capabilities of GOALL-Epitaxy, the implications of these advancements reach far beyond mere material synthesis. The potential applications of these artificial oxide structures could redefine the landscape of electronic devices, harnessing new functionalities that could lead to faster, more efficient technologies. With every layer meticulously constructed, the scientific community watches eagerly as the intricate tapestry of materials science unfolds, driven by the relentless pursuit of knowledge and innovation.

The journey of GOALL-Epitaxy illustrates the intricate interplay between scientific curiosity and technological advancement. As researchers unwrap the layers of complexity inherent in materials like transition metal oxides, the possibilities become boundless. The innovative hybridization of techniques, coupled with a keen understanding of material properties, demonstrates how interdisciplinary approaches can lead to groundbreaking discoveries. This research not only stands to elevate the capabilities within materials science but also resonates with broader implications for energy, electronics, and beyond.

In essence, Gigantic-Oxidative Atomic-Layer-by-Layer Epitaxy (GOALL-Epitaxy) emerges as a beacon of hope for scientists navigating the intricacies of oxide materials. By overcoming long-standing challenges associated with existing techniques, this novel methodology heralds a new era in the exploration of complex oxides and their applications, paving the way for future breakthroughs in the realm of strongly correlated electronic systems.

This cutting-edge advancement encapsulates an inspiring narrative of perseverance and ingenuity, propelling forward the field of materials science. Researchers remain undeterred in their quest for deeper understanding and more sophisticated methodologies, confident that every breakthrough brings humanity one step closer to unlocking the full potential of advanced materials. The science community is invited to engage with this exciting frontier, where the synthesis of intricate materials echoes across disciplines, promising a future laden with innovation and discovery.

Subject of Research: Growth of oxide materials using Gigantic-Oxidative Atomic-Layer-by-Layer Epitaxy (GOALL-Epitaxy)
Article Title: Gigantic-oxidative atomic-layer-by-layer epitaxy for artificially designed complex oxides
News Publication Date: 2025
Web References: National Science Review
References: Original research published in National Science Review
Image Credits: ©Science China Press

Keywords

Transition metal oxides, GOALL-Epitaxy, high-temperature superconductivity, materials science, layered oxide structures, thin-film growth techniques, pulsed laser deposition, molecular beam epitaxy, complex oxides, strongly correlated electronic systems.

Hydrogen sulfide, a simple molecule when composed as H₃S under high pressures, made headlines several years ago owing to its remarkable superconducting transition temperature (T_c) exceeding 200 K. This stunning discovery created a paradigm shift by illustrating that conventional electron-phonon mechanisms, long thought incapable of producing superconductivity at such formidable scales, might indeed underlie these unprecedented critical temperatures. However, despite theoretical predictions and indirect experimental indications, the direct spectroscopic characterization of the superconducting gap — a hallmark of the ordered state — remained elusive until now.

The superconductor’s energy gap, often dubbed the order parameter, embodies the energy scale at which electrons pair up to form Cooper pairs and condense into a superconducting phase. Detecting and measuring this gap with high precision stands as a cornerstone to confirming the nature of superconductivity, distinguishing between conventional phonon-mediated mechanisms and more exotic pairing scenarios such as those involving spin fluctuations or unconventional symmetries. In the case of hydrogen sulfide, tunneling spectroscopy, which probes electronic states near the Fermi level with exquisite energy resolution, has successfully captured this spectral fingerprint for the first time.

According to the recent report by Du, Drozdov, Minkov, and collaborators, tunneling measurements revealed a superconducting gap value of approximately 30 millielectronvolts (meV) for the H₃S sample designated as S1. This value, although substantial, intriguingly falls below the magnitude anticipated by current theoretical frameworks, which had predicted larger gap amplitudes based on strong electron-phonon coupling models. Additionally, the so-called 2Δ/k_BT_c ratio — a dimensionless parameter that scales the gap with respect to the critical temperature — was measured at 3.54, aligning closely with the classic Bardeen-Cooper-Schrieffer (BCS) theory expectation for weak to moderate coupling superconductors.

This apparent discrepancy between empirical data and theoretical predictions opens a compelling dialogue within the superconductivity community. While the isotope effect observed, involving substitution with deuterium to form D₃S, and the consistency with an s-wave symmetry gap strongly point towards phonon-driven pairing interactions, the reduced gap magnitude invites more nuanced scrutiny. This could suggest that current theoretical treatments, though sophisticated, might yet lack the full complexity of the actual interactions or structural inhomogeneities present in these pressurized samples.

Moreover, the investigations uncovered evidence of a multigap superconducting scenario within inhomogeneous hydrogen sulfide specimens. Multigap superconductivity, wherein distinct gaps coexist on different parts of the Fermi surface or in spatially segregated superconducting phases, represents a richer and more intricate state than single-gap models. Its presence here highlights the heterogeneous nature of high-pressure hydrides and the necessity for comprehensive studies addressing phase separation, crystal structure variations, and their influence on superconducting parameters.

The implications of these findings extend far beyond hydrogen sulfide alone. The successful application of tunneling spectroscopy to dissect the superconducting gap in such challenging experimental conditions underscores the technique’s potency as a diagnostic tool. It paves the way for analogous explorations across the broader family of metal superhydrides and related materials. Understanding their superconducting gap structures with high fidelity is vital for decoding the mechanisms responsible for their often remarkably high critical temperatures and for guiding the discovery of new compounds operable at lower pressures.

These developments dovetail with a wider scientific pursuit to link microscopic interactions with macroscopic superconducting properties. Resolving the nature of electron-phonon coupling strength, anisotropies in the order parameter, or the presence of competing phases can decisively inform the direction of theoretical modeling and synthetic efforts. Furthermore, such insight contributes to the ultimate goal of engineering materials capable of attaining room-temperature superconductivity under ambient or technologically feasible conditions.

The interplay between theory and experiment witnessed in this work exemplifies the dynamic evolution of superconductivity research. The direct experimental detection of a superconducting gap, particularly with high resolution and under multi-megabar pressures, sets a new benchmark. Yet it also raises new questions, such as the precise origin of the discrepancy between observed and predicted gap magnitudes and the full character of the multigap phenomena manifesting in these complex hydride systems.

Expanding the scope of this research to include systematic isotopic substitution and pressure-dependent studies could unravel further subtleties governing pairing interactions. Additionally, complementary spectroscopic techniques like angle-resolved photoemission or neutron scattering may illuminate collective excitations and electronic structure modifications concomitant with superconductivity. Such multi-pronged approaches will build a more comprehensive picture of these enigmatic states.

In sum, the first unambiguous tunneling spectroscopic identification of the superconducting gap in hydrogen sulfide marks a seminal advance in high-pressure superconductivity science. It solidifies the central role of phonon-mediated Cooper pairing in sustaining superconductivity at record-high temperatures within this material family. Simultaneously, the nuanced deviations from theoretical expectations beckon deeper inquiries into material-specific complexities and the quest for materials surpassing current performance benchmarks.

As experimental methods continue to improve and theoretical models become increasingly sophisticated, the path toward realizing superconductors functional at ambient conditions grows ever clearer. Hydrogen sulfide and its hydride cousins today exemplify the remarkable achievements borne from this synergy of experimental innovation and intellectual exploration, promising a future where lossless electrical conduction could revolutionize energy, transportation, and beyond.

Subject of Research: Superconductivity and superconducting gap characterization in high-pressure hydrogen sulfide (H₃S).

Article Title: Superconducting gap of H₃S measured by tunnelling spectroscopy.

Article References:
Du, F., Drozdov, A.P., Minkov, V.S. et al. Superconducting gap of H₃S measured by tunnelling spectroscopy. Nature (2025). https://doi.org/10.1038/s41586-025-08895-2

Image Credits: AI Generated