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.

Graphene consists of a single layer of carbon atoms arranged in a two-dimensional honeycomb lattice. In an ordinary sheet, electrons can move with exceptional mobility, behaving approximately like massless particles over a broad energy range. However, when one graphene layer is placed on top of another and rotated by a small angle, the two atomic lattices interfere to create a much larger periodic pattern known as a moiré superlattice. At a critical rotation of approximately 1.1 degrees, known as the magic angle, the electronic bands become extremely narrow and nearly flat. This flattening dramatically reduces the kinetic energy available to electrons, allowing their mutual Coulomb repulsion to dominate the system’s behavior.

The result is a highly tunable platform in which electrons can no longer be treated as independent particles. Instead, their collective interactions can produce insulating phases even when conventional band theory would predict metallic behavior. Experiments on magic-angle twisted bilayer graphene have revealed correlated insulating states, superconductivity, orbital magnetism and quantum anomalous Hall behavior. The review’s authors, Assistant Professor Shuo-Ying Yang of the Southern University of Science and Technology and Professor Cheng Shen of the University of Electronic Science and Technology of China, describe these phenomena as connected consequences of the same central design principle: twisting graphene reshapes its electronic structure until correlation and topology become impossible to ignore.

Flat electronic bands are particularly important because they concentrate many electronic states within a narrow energy window. In a dispersive band, electrons can lower their energy by moving through the crystal, and this kinetic energy often competes successfully with interactions. In a flat band, that motion is strongly suppressed. Even relatively modest Coulomb interactions can therefore reorganize the electrons into ordered states. These may include correlated insulators, valley-coherent phases and unusual “heavy-fermion-like” states in which charge carriers appear to acquire a greatly enhanced effective mass. The valley degree of freedom, associated with distinct energy extrema in graphene’s band structure, provides an additional internal label that can participate in this ordering.

Twisted graphene is not only a system of strong electronic correlation; it is also a system with unusual quantum geometry. The wave functions in its flat bands can possess nontrivial Berry curvature, a geometric property of quantum states that acts in some ways like a magnetic field in momentum space. Berry curvature and related band-topological characteristics can generate orbital magnetic moments and support phases such as orbital Chern insulators. In these states, electrons collectively occupy bands with a nonzero topological invariant, enabling conducting edge channels even when the bulk is insulating. Under suitable conditions, the system can also display a quantum anomalous Hall effect, in which electrical current flows along the edges without an externally applied magnetic field.

The combination of topology and electron interaction makes the resulting phases especially rich. In conventional materials, topology is often discussed in terms of relatively weakly interacting electrons, while correlation is treated as a separate source of complexity. Magic-angle graphene brings the two effects together in a clean, adjustable structure. Changes in carrier density, electric displacement field, pressure, magnetic field or twist angle can shift the balance among competing phases. This tunability allows researchers to explore how topological order, symmetry breaking and electronic correlation emerge, compete and sometimes coexist within the same material platform.

Superconductivity is one of the most closely watched consequences of this competition. When a material becomes superconducting, electrons form collective paired states that can carry electrical current without resistance. In conventional Bardeen–Cooper–Schrieffer theory, these pairs are typically produced by interactions involving lattice vibrations. However, observations in twisted graphene have increasingly suggested that its superconductivity may not fit neatly within this conventional picture. The relatively low carrier densities, proximity to correlated insulating states and sensitivity to the system’s internal quantum structure all point toward a strong-coupling and potentially unconventional pairing mechanism.

Quantum geometry may also help explain how superconductivity survives in a flat-band system. A simple flat band appears unfavorable for superconductivity because the usual contribution from electron velocity to superfluid stiffness is strongly reduced. Yet the geometry of the electronic wave functions can provide an additional geometric contribution to that stiffness. This contribution can support phase-coherent superconductivity even when the bands themselves have very little dispersion. The idea offers a possible explanation for why superconducting behavior can emerge from electronic structures that seem, at first glance, unable to sustain the movement needed for a robust superfluid state.

The review further explores how researchers are extending the original bilayer design into more elaborate architectures. Multilayer systems with different numbers of graphene sheets, alternating-twist structures and supermoiré materials can produce several interfering length scales and more intricate band structures. These platforms offer new ways to control bandwidth, topology, layer polarization and interaction strength. As fabrication techniques improve, the number of accessible quantum phases is expected to grow, potentially enabling controlled transitions between correlated metals, insulators, magnetic states, topological phases and unconventional superconductors.

Twisted graphene has therefore evolved from an elegant demonstration of moiré physics into a broad research frontier linking materials science, quantum geometry and many-body physics. Its appeal lies not only in the remarkable states already observed, but also in the ability to engineer them through a geometric parameter measured in degrees. The emerging picture is that rotation can act as a form of quantum control, converting ordinary carbon sheets into programmable environments for discovering new collective behavior. As moiré engineering and measurement technologies advance, twisted graphene and related systems could provide both fundamental insights into quantum matter and a foundation for future electronic, magnetic and quantum-device applications.

Subject of Research: Twisted graphene moiré superlattices, electronic correlation, topology and unconventional superconductivity

Article Title: “Twisting Graphene into Correlation and Topology”

Web References: https://doi.org/10.1093/nsr/nwag363

References: National Science Review, DOI: 10.1093/nsr/nwag363

Image Credits: ©Science China Press

Keywords: magic-angle twisted bilayer graphene, twistronics, moiré superlattices, flat bands, correlated electrons, quantum geometry, Berry curvature, quantum anomalous Hall effect, topological phases, unconventional superconductivity

Materials derive their fundamental characteristics from the dynamic behavior of their constituent particles. Electrons dictate electrical conductivity, while phonons — quantized vibrations of the atomic lattice — govern thermal transport. When these electrons and phonons interact, the resulting coupling can give rise to groundbreaking quantum phenomena. Among the most compelling of these is superconductivity — a state marked by zero electrical resistance — often triggered by phonon-mediated electron pairing. Yet, the difficulty in directly measuring how electrons couple to each individual phonon mode has long impeded deeper insights into these mechanisms.

The original Quantum Twisting Microscope, devised two years ago by the research team led by Professor Shahal Ilani, harnessed the properties of atomically thin van der Waals materials as quantum interferometers at its probe tips. Operating at room temperature, this instrument could image electronic wavefunctions with remarkable spatial resolution, mapping the electronic spectra of diverse quantum materials. However, its capabilities to directly resolve the subtle lattice vibrations remained unattainable — until now.

The newly developed cryogenic QTM operates at ultra-low temperatures, enhancing its sensitivity and heralding a paradigm shift in the imaging of phonons. It exploits an inelastic tunneling process between two atomically-thin layers, where electrons passing through emit phonons with precisely controlled energies and momenta. By finely adjusting the voltage bias and the twist angle between the layers, researchers can systematically tune and scan a wide portion of the phonon energy landscape, mapping its complete spectrum in extraordinary detail.

This precise control and detection method illuminate not only the presence of unique phonon modes but also how strongly electrons couple to each of these modes individually. As Dr. John Birkbeck explains, “Our technique transcends traditional phonon spectroscopy by providing quantitative measurements of the electron-phonon coupling strength at the single-mode level across a broad momentum range.” This affords unprecedented insight into the fundamental dynamics underpinning quantum behavior in advanced materials.

The application of this technique to twisted bilayer graphene led to a remarkable and unforeseen discovery: the identification of a distinctive low-energy collective excitation termed the “phason.” Unlike typical phonons, phasons are associated with the relative sliding motion between the two graphene sheets. Notably, the electron-phason coupling intensifies as the twist angle approaches the celebrated magic angle, a configuration known to produce exotic superconducting and strange metallic phases. This link hints that phasons may be central actors in the emergence of these quantum states.

Beyond phonons and phasons, the versatility of the cryogenic QTM promises to open new investigative frontiers. Co-author Jiewen Xiao highlights that the method is broadly applicable to the detection of any collective excitation that couples to tunneling electrons. This capability positions the microscope as a vital tool to probe plasmons, magnons, spinons, and other Goldstone modes within a variety of quantum materials, dramatically expanding our experimental toolkit for condensed matter physics.

As we increasingly seek to unravel the mysteries of quantum materials, tools like the cryogenic QTM become indispensable. The research team, including lead author Alon Inbar, expresses optimism that this technical innovation will catalyze rapid progress in understanding the intricate coupling mechanisms at play and unlock new quantum phases of matter that have thus far eluded comprehensive experimental observation.

The cryogenic QTM’s dual capacity to image both the electronic states and their coupled collective excitations crucially positions it at the intersection of fundamental research and applied quantum technologies. Insights gleaned from this instrument are anticipated to accelerate advancements in quantum computing, high-precision sensing, and emerging quantum electronic devices, where harnessing such intricate electron-boson interactions is essential.

The full implications of this research are vast and ripple across the fields of material science and condensed matter physics. By enabling mode-selective and momentum-resolved measurements of electron-phonon interactions, the QTM facilitates an unparalleled understanding of superconductivity’s microscopic origins and the exotic metallic states that challenge current physics paradigms. This opens doors to engineering materials with custom quantum properties tailored for future technologies.

In summary, the introduction of the cryogenic Quantum Twisting Microscope marks a quantum leap in our investigative capabilities. Its application to twisted bilayer graphene reveals that phasons, this newly observed quantum vibrational mode, may play an essential role in modulating the quantum phases within these atomically engineered structures. As this technology matures, it stands poised not only to deepen our comprehension of existing quantum phenomena but also to uncover entirely new realms of quantum matter.

With every new measurement facilitated by QTM, we get closer to unraveling the complex tapestry of interactions that dictate the behavior of electrons in quantum materials. The researchers’ exploration foreshadows a new era where detailed spectroscopic mapping of collective modes becomes routine, laying the foundation for discoveries that could redefine our technological landscape.

Subject of Research:
Electron-phonon coupling and collective excitations in twisted bilayer graphene studied via cryogenic Quantum Twisting Microscopy.

Article Title:
Quantum twisting microscopy of phonons in twisted bilayer graphene

News Publication Date:
2025

Web References:
Not specified in the source material.

Image Credits:
Not specified in the source material.

Keywords

Phonons, Graphene, Basic research, Discovery research, Superconductivity, Low temperature physics, Measuring instruments, Vibration