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.

Topological superconductors are of great interest due to their ability to host Majorana fermions, quasiparticles anticipated to play a fundamental role in fault-tolerant quantum computing. Among these, Weyl topological superconductors stand out as they feature unique momentum space characteristics: point nodes called Weyl points, which act as monopoles and anti-monopoles of Berry curvature, connected by Fermi arc states on the surface. Realizing these systems experimentally has remained a challenge owing to the need for subtle relativistic effects and intricate electronic structures.

The key insight from this new work lies in the concept of geometric frustration in three-dimensional lattices. While in two-dimensional square lattices, electrons with repulsive interactions favor pairing states with a d-wave symmetry that changes sign between x and y directions, extending this idea to three dimensions leads to a mathematical contradiction. Specifically, it is impossible for the superconducting order parameter to flip sign along all three orthogonal axes simultaneously without frustration.

To resolve this conundrum, the system naturally adopts a chiral d+id pairing state that circumvents conventional sign-flipping. Instead of sharp phase changes of 0 or 180 degrees, the superconducting phase smoothly rotates by 120 degrees among the x, y, and z directions. This continuous phase winding avoids frustration and stabilizes the superconducting order.

Unlike conventional chiral superconductors that exhibit net orbital angular momentum, this distinct 120-degree phase structure generates an octupolar symmetry. This symmetry is intimately linked to the emergence of eight Weyl nodes located along the body diagonals of the crystal lattice. These nodes alternate in topological charge, behaving as positive and negative magnetic monopoles in momentum space. The resulting state is aptly named an octupolar Weyl superconductor, reflecting the intricate topology of its gap nodes.

By demonstrating that spin-orbit coupling is not a prerequisite for Weyl topological superconductivity, this research opens the door to a broader range of candidate materials, including strongly correlated cubic compounds. Additionally, the findings provide a compelling platform for exploring unconventional superconductivity and topological phases in three-dimensional settings. The theoretical models also suggest that cold-atom quantum simulations could serve as versatile experimental testbeds for probing this novel state.

This discovery significantly advances our theoretical understanding and practical quest for Weyl superconductors, offering fresh opportunities to harness their exotic quasiparticles for next-generation quantum technologies. As experimental techniques evolve, the search for octupolar Weyl topological superconductors may soon transition from theory to reality, potentially revolutionizing quantum materials science.

Subject of Research: Topological superconductivity, Weyl superconductors, electron interactions in cubic lattices
Article Title: Octupolar Weyl Superconductivity Emerging from Electron Interaction Frustration in 3D Cubic Lattices
Web References: http://dx.doi.org/10.1093/nsr/nwag326
Image Credits: ©Science China Press
Keywords: Weyl topological superconductor, octupolar superconductivity, chiral d+id pairing, geometric frustration, Majorana fermions, quantum computing, Bogoliubov nodes, cubic lattice

Coulomb drag is an inherently fascinating phenomenon where the movement of charged particles, or current, in one conductor can induce a voltage in a nearby but electrically isolated conductor. This interaction arises purely through long-range Coulomb forces — the electrostatic repulsion or attraction between charged particles — without any direct electrical contact. Previous studies have characterized Coulomb drag extensively in conventional two-dimensional electron systems, but exploring this effect in topological materials marked by non-trivial band structures has remained an elusive challenge until now.

Chern insulators represent a unique class of magnetic topological materials distinguished by their capacity to exhibit the quantum anomalous Hall effect (QAH). Unlike the classic quantum Hall effect, which necessitates external magnetic fields, Chern insulators display quantized Hall conductance due to intrinsic magnetization combined with robust chiral edge states that allow dissipationless transport along their boundaries. These edge modes are resilient to disorder and scattering, making Chern insulators prime candidates for applications in spintronics and quantum information.

The significance of this research lies not only in the pioneering observation of a non-reciprocal Coulomb drag effect but also in its implications for the control and detection of quantum states in advanced materials. Non-reciprocal phenomena, where the physical response depends on the direction of applied stimuli, are of increasing interest because they can enable new electronic functionalities, such as rectification and isolation, fundamental to quantum circuits and devices. By demonstrating such asymmetry in Coulomb drag, the research reveals intricate coupling mechanisms between quantum edge states mediated by Coulomb interactions.

To execute these experiments, the team employed molecular beam epitaxy (MBE) to grow ultrathin films of vanadium-doped (Bi,Sb)₂Te₃, a prototypical topological insulator system chemically engineered to promote a high-temperature quantum anomalous Hall effect. Utilizing a dual Hall-bar device architecture separated by a nanoscale vacuum gap ensured that coupling between layers occurred exclusively through Coulomb forces, eliminating unwanted tunneling currents that could mask the pure electrostatic interaction signals. This meticulous device design allowed precise probing of Coulomb drag dynamics under stringent experimental conditions.

Measurements were conducted at ultra-low temperatures reaching as low as 20 millikelvin and under perpendicularly applied magnetic fields to investigate the detailed interplay of magnetization and quantum transport phenomena. The researchers recorded both longitudinal (along current direction) and transverse (perpendicular to current flow) drag voltages, supplementing these with current-voltage (I-V) characterizations to differentiate between shot noise and mesoscopic fluctuation regimes. Temperature-dependent scaling analysis further confirmed the mesoscopic origins of the observed behaviors.

One of the most striking findings was the fixed polarity of longitudinal drag signals regardless of the current direction or magnetic field polarity. This rectification-like property indicates an inherent directionality in Coulomb drag, breaking conventional expectations of reciprocal behavior in electronic transport. Conversely, the transverse drag exhibited a clear dependence on the magnetization’s orientation, pinpointing the role of chiral edge state couplings between the layers as the dominant conduit for non-reciprocal interactions.

Delving into the underlying mechanisms, the study identified mesoscopic fluctuations as the primary factor influencing Coulomb drag at ultra-low temperatures, with a characteristic quadratic temperature dependence (T²). As bias currents increased, shot noise—quantum noise intrinsic to discrete charge carriers—became the prevailing driver, introducing nonlinearities in the drag voltages that correspond to changes in quantum transport regimes. This duality underscores the rich complexity of electron correlations in topological insulator systems and opens avenues for tuning device responses by controlling temperature and bias conditions.

Beyond fundamental physics, these insights have profound implications for the rapidly advancing field of topological quantum computing. The non-contact detection technique introduced here provides a sensitive probe for quantum states, particularly those relevant to qubit operations based on Majorana fermions and other exotic quasiparticles. The ability to monitor quantum coherence and state transitions without perturbing fragile quantum information is a critical milestone toward scalable and robust quantum technologies.

Moreover, the asymmetric Coulomb drag effect uncovered in Chern insulators could inspire innovative device architectures that leverage magnetization dynamics to enable low-power, chiral electronic components. Devices exploiting such directional coupling could revolutionize spintronic circuits, offering new pathways to integrate magnetic control with topological robustness for improved performance and energy efficiency.

This breakthrough underscores the power of combining cutting-edge materials science with precision quantum transport measurements to unlock unforeseen physical phenomena. By charting previously unexplored territory in non-reciprocal Coulomb drag, He Qinglin’s group has expanded our comprehension of topology-driven quantum interactions and set the stage for future explorations that may transform quantum electronics and computation.

The publication of this work in Nature Communications attests to its significance within the physics community and its potential impact across multiple domains including condensed matter physics, quantum materials engineering, and information science. As researchers worldwide build on these findings, this report will stand as a seminal contribution highlighting the interplay of topology, magnetism, and Coulomb interactions in quantum materials.

Pioneering experimental techniques, such as the dual Hall bar nanoscale gap device employed by the team, illustrate the meticulous engineering necessary to study subtle quantum effects. This approach could be adapted to investigate other topological phases or explore dynamic control of quantum states via external stimuli. The synergy between intrinsic material properties and novel measurement strategies signals a vibrant future for the field.

In sum, the first observation of non-reciprocal Coulomb drag in magnetic Chern insulators marks a milestone that bridges fundamental quantum physics and emerging quantum technology. This achievement expands the horizon for identifying and harnessing new quantum phenomena where topology, symmetry breaking, and electron correlations converge, paving the way for breakthroughs in understanding and utilizing complex quantum systems.

Subject of Research: Observation and analysis of non-reciprocal Coulomb drag phenomena in magnetic Chern insulators exhibiting quantum anomalous Hall effects.

Article Title: Non-reciprocal Coulomb drag between Chern insulators

News Publication Date: April 24, 2025

Web References:

• Nature Communications article DOI: 10.1038/s41467-025-58401-5
• Full article (PDF)

Keywords

Topology, Quantum states, Quantum anomalous Hall effect, Chern insulators, Coulomb drag, Quantum materials, Mesoscopic fluctuations, Shot noise, Non-reciprocal transport, Majorana qubits, Molecular Beam Epitaxy, Quantum computing