The puzzle begins with a simple expectation. When a material enters the superconducting state, electrical resistance should drop to exactly zero as the temperature falls. Yet in a growing family of two-dimensional superconductors — including gate-tuned graphene systems — researchers have repeatedly observed regions where the resistance falls sharply but then saturates at a finite value instead of vanishing. These states show abrupt transitions out of the superconducting-like regime when temperature, magnetic field or current exceed critical thresholds, mimicking the phenomenology of true superconductivity. But the resistance never reaches zero. Theoretical physicists proposed in 1999 that such a ‘Bose metal’ could exist as a genuine quantum ground state, and a 2019 colloquium review catalogued the anomalous metal as a ‘failed superconductor’ seen in disordered films since the late 1980s. What has been missing is a clean, well-controlled system in which the state can be studied systematically.

The new experiment delivers exactly that. The team, led by Matthew Yankowitz at the University of Washington and Joshua Folk at the University of British Columbia, built devices from rhombohedral graphene — a stacked form of carbon in which the layers are aligned in the same lateral registry rather than the staggered arrangement of ordinary graphite. This stacking order produces flat electronic bands that amplify interactions between electrons, and rhombohedral graphene has in recent years become a prolific source of unconventional superconductivity. The researchers encapsulated their graphene in hexagonal boron nitride crystals supplied by Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Japan, and placed it on a tungsten diselenide substrate that imparts strong spin–orbit coupling to the carbon electrons. Dual electrostatic gates allowed them to tune both the carrier density and the perpendicular electric field across the device with exquisite precision.

What they found was striking. At zero magnetic field, mapping the resistance across the full two-dimensional gate-voltage landscape revealed isolated pockets where the resistance dropped to zero — bona fide superconducting islands — embedded in a broader region of gate space where the resistance instead saturated at a finite value. Crucially, the two kinds of pockets displayed nearly identical phenomenology. Both showed sharp, abrupt transitions to the normal state as temperature was raised, as a perpendicular magnetic field was applied, or as the driving current was increased beyond a critical value. To any casual inspection, the two regimes looked like siblings. One simply carried current perfectly; the other did not.

The most dramatic twist came when the researchers applied a small magnetic field parallel to the plane of the graphene sheet. In conventional superconductors, an in-plane field has little effect because the field lines slip between the atomically thin layers without generating vortices. Here, however, a modest parallel field of a few hundred millitesla expanded the finite-resistance pockets and merged them with their zero-resistance neighbours, without qualitatively changing their behaviour. The result was a razor-sharp boundary, at millikelvin base temperature, between states of zero and finite resistance carved directly into the gate-voltage map. The superconducting pockets shrank while the anomalous-metal territory grew, suggesting that the parallel field gently tips the balance between the two competing phases rather than destroying either one outright.

The finite-resistance state reproduced the key experimental signatures that define the anomalous metal in thin-film superconductors. Most tellingly, the team measured a non-monotonic dependence on current: the differential resistance of the state varied in a complex, non-monotonic way as the applied direct current was swept, a hallmark previously documented in disordered films and Josephson-junction arrays. The researchers also performed a Berezinskii–Kosterlitz–Thouless analysis, examining how the voltage scales with current near the transition. In the true superconducting pocket, the current–voltage curves followed the expected cubic power law over a broad range, yielding a transition temperature of about 61 millikelvin. In the anomalous-metal pocket, the same criterion was never satisfied — the state flirts with superconducting order but never locks it in.

Why does this matter so much? The anomalous metal has been observed in quench-condensed aluminium films, granular metals, amorphous molybdenum–germanium films, electrostatically doped cuprate films, ion-gated two-dimensional superconductors, and hybrid semiconductor–superconductor Josephson arrays. In every previous case, the material contained substantial disorder, and skeptics could always argue that the finite resistance arose from sample-specific imperfections, inhomogeneous current paths, or heating effects rather than a genuine quantum state of matter. Ultra-clean rhombohedral graphene, with carrier mobility far exceeding that of any thin film and gate-tunable parameters that can be swept continuously, places severe constraints on all such extrinsic explanations. The state here is not an artifact of dirty metallurgy; it is an intrinsic property of a pristine quantum material.

The theoretical implications are equally profound. Proposed explanations for the anomalous metal invoke quantum fluctuations of the superconducting phase, dissipation from normal quasiparticles, a glassy tangle of superconducting vortices, or microscopic inhomogeneity that percolates superconducting islands into a resistive network. Each model makes different predictions about how the state should respond to magnetic field, current and disorder. The rhombohedral graphene platform, in which the superconducting and anomalous-metal pockets can be relocated, expanded and merged at will by turning gate voltages and applying small in-plane fields, offers a testing ground in which those predictions can be discriminated for the first time. The fact that the two phases share nearly identical critical behaviour — the same abrupt transitions, the same response to temperature and field — hints that they may be two faces of a single underlying quantum phase transition, separated by a boundary whose sharpness at base temperature suggests a genuine quantum critical line.

There is also a broader resonance with the wider graphene superconductivity programme. Since the 2018 discovery of unconventional superconductivity in magic-angle twisted bilayer graphene, the carbon family has yielded spin-polarized superconductors, chiral superconducting states, record violations of the Pauli paramagnetic limit, and superconductivity intertwined with ferroelectric orbital magnetism and topological charge order. The new result adds a crucial missing character to this cast: the anomalous metal, long confined to the margins of thin-film physics, now sits squarely within the same gate-tunable phase diagrams that host the most celebrated unconventional superconductors. Understanding why some pockets superconduct and others fail to do so — separated by nothing more than a small change in carrier density or electric displacement field — may reveal the microscopic ingredient that tips the balance, whether it be spin polarization, isospin texture, or the delicate interplay of flat-band kinetics and Coulomb repulsion.

For now, the anomalous metal remains without an accepted theoretical explanation, but the conditions of the hunt have fundamentally changed. A state once accessible only in disordered films of variable quality can now be dialled up on demand in an atomically clean, electrostatically programmable material, measured reproducibly across multiple devices and contact pairs, and probed with the full modern toolkit of quantum transport. As the authors and their collaborators — including theorists Boris Spivak, Steven Kivelson and Aharon Kapitulnik, whose work frames the field — continue to explore this system, the failed superconductor may soon give up its secrets. If it does, the answer will reshape our understanding not only of two-dimensional superconductivity but of the quantum phase transitions that govern how collective electronic order emerges, or collapses, at the coldest temperatures physics can reach.

Subject of Research: Anomalous metal and superconducting phases in rhombohedral graphene on a WSe2 substrate

Article Title: Anomalous metal and superconducting phases in rhombohedral graphene

Article References: Okounkova, A., Sohm, A., Faehndrich, T., Kumar, M., Waleffe, D., Yan, J., Watanabe, K., Taniguchi, T., Folk, J., & Yankowitz, M. (2026). Anomalous metal and superconducting phases in rhombohedral graphene. Nature. https://doi.org/10.1038/s41586-026-11033-1

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11033-1

Keywords: rhombohedral graphene, anomalous metal, superconductivity, two-dimensional materials, quantum phase transition, gate tuning, spin–orbit coupling, Berezinskii–Kosterlitz–Thouless transition, failed superconductor, WSe2, quantum transport, flat bands

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