Conductors

Researchers from Brown University, Harvard University and National Institute for Materials Science (Tsukuba) have reported new insights into how superconductivity, electronic nematicity and strange metallicity are connected in 'magic-angle' twisted trilayer graphene, using angle-resolved transport measurements to track how these phases evolve.

Superconductivity - where electrical resistance drops to zero - is often linked to a breaking of rotational symmetry in the electronic system, known as nematicity. At the same time, many materials already show directional differences in electrical transport (anisotropy) before becoming superconducting. This has made it difficult to determine whether the symmetry breaking in the superconducting state is intrinsic, or simply inherited from the normal metallic phase. To address this, the researchers developed a method that measures electrical resistance as a function of direction. Instead of probing transport along a single axis, they continuously rotated the direction of current flow and mapped how resistance changes with angle. This allowed them to directly compare the angular behavior of three closely related states: the normal metallic phase, the superconducting phase, and the so-called strange metal phase, which exhibits unconventional temperature-dependent resistance.

Researchers from Ohio State University, Imdea Nanoscience and the National Institute for Materials Science in Tsukuba have demonstrated that superconductivity in twisted bilayer graphene (tBLG) can be tuned - and even completely switched off - by engineering its dielectric environment. Their work reveals that, unlike in conventional phonon-mediated superconductors, the pairing mechanism in this moiré system is strongly controlled by electronic interactions that are highly sensitive to nearby materials.

In the study, the team fabricated twisted bilayer graphene devices and positioned them a few nanometers above a bulk strontium titanate (SrTiO₃) substrate, a synthetic perovskite often referred to as a man‑made “diamond” because of its robustness and very large, tunable dielectric constant. By increasing this dielectric constant in situ, they steadily suppressed both the height and the width of the superconducting dome in magic‑angle devices and, upon further tuning, extinguished superconductivity altogether across the entire dome. At larger twist angles, where devices on standard SiO₂ substrates typically do not superconduct, the SrTiO₃ environment enabled a superconducting “pocket” even in regimes where correlated insulating states were absent, underscoring how delicately the phase diagram depends on dielectric screening.

A research team led by Ecole Polytechnique Fédérale de Lausanne (EPFL) has discovered that a “supermoiré lattice” graphene arrangement shows superconductivity and causes electrons to behave in unusual, coordinated ways. The findings could help design new kinds of quantum materials with properties never seen before.

When two graphene sheets are rotated relative to each other, they form what are known as moiré lattices. At certain “magic” twist angles, the electrons in these moiré lattices interact strongly, giving rise to superconductivity and insulating behavior reminiscent of that found in high-temperature superconductors. Less is known, however, about what happens when multiple moiré lattices overlap and create an even larger and even more complex ("higher order") pattern: a supermoiré lattice. 

Researchers form MIT and Japan's National Institute for Materials Science have observed key evidence of unconventional superconductivity in “magic-angle” twisted tri-layer graphene (MATTG) - a material that is made by stacking three atomically-thin sheets of graphene at a specific angle, or twist, that then allows exotic properties to emerge.

MATTG has shown indirect hints of unconventional superconductivity and other strange electronic behavior in the past. The new discovery offers the most direct confirmation yet that the material exhibits unconventional superconductivity. Specifically, the team was able to measure MATTG’s superconducting gap - a property that describes how resilient a material’s superconducting state is at given temperatures. They found that MATTG’s superconducting gap looks very different from that of the typical superconductor, meaning that the mechanism by which the material becomes superconductive must also be different, and unconventional.

A research team, led by Daniil Gorbachev and Na Xin at the University of Manchester and working with colleagues including Kenji Watanabe and Takashi Taniguchi, demonstrated a major improvement in graphene’s electronic properties by strategically positioning graphite gates in extremely close proximity to the material. 

This innovative approach, which involves placing the gates just one nanometer away, dramatically reduces charge variations and potential fluctuations, ultimately boosting graphene’s mobility to exceed even the highest-quality semiconductor heterostructures. The resulting material exhibits exceptional performance, enabling the observation of subtle quantum phenomena previously hidden by disorder and paving the way for a new era in two-dimensional materials research.

In systems with multiple energy bands, the interplay between electrons with different effective masses drives correlated phenomena that do not occur in single-band systems. Magic-angle twisted trilayer graphene is a tunable platform for exploring such effects, hosting both heavy ("bound") electrons and light ("weakly bound and mobile") electrons. 

Researchers at Harvard, MIT and National Institute for Material Science in Japan have examined the interplay between "light" and "heavy" electrons in magic-angle twisted trilayer graphene, shedding new light on how they may help form novel quantum states.

Researchers from MIT, University of Basel, Florida State University and National Institute for Materials Science in Japan have reported a "chiral superconductor" - a rhombohedral tetra- and penta-layer graphene material that conducts electricity without resistance, and is also, paradoxically, intrinsically magnetic. The team has found that when four or five sheets of graphene are stacked in this "rhombohedral" configuration, the resulting structure can exhibit exceptional electronic properties that are not seen in graphite as a whole.

In their new study, the scientists isolated microscopic flakes of rhombohedral graphene from graphite, and subjected the flakes to a battery of electrical tests. They found that when the flakes are cooled to 300 millikelvins (about -273 degrees Celsius), the material turns into a superconductor, meaning that any electrical current passing through the material can flow through without resistance. They also found that when they swept an external magnetic field up and down, the flakes could be switched between two different superconducting states, just like a magnet. This suggests that the superconductor has some internal, intrinsic magnetism. Such switching behavior is absent in other superconductors.

Researchers from MIT and Japan's National Institute for Materials Science have directly measured superfluid stiffness for the first time in “magic-angle” graphene — two or more atomically thin sheets of graphene twisted with respect to each other at just the right angle to enable a host of exceptional properties, including unconventional superconductivity. The term “superfluid stiffness,” or the ease with which a current of electron pairs can flow, is a key measure of a material’s superconductivity.

This superconductivity makes magic-angle graphene a promising building block for future quantum-computing devices, but exactly how the material superconducts is not well-understood. Knowing the material’s superfluid stiffness will help scientists identify the mechanism of superconductivity in magic-angle graphene. The team’s measurements suggest that magic-angle graphene’s superconductivity is primarily governed by quantum geometry, which refers to the conceptual “shape” of quantum states that can exist in a given material.

Researchers at Korea's Pohang University of Science and Technology and Japan's National Institute for Materials Science have developed a way to control the properties of graphene by combining superconductors and graphene. 

Professor Lee Gil-ho of Pohang University of Science and Technology (POSTECH) and researchers from the Research Institute, in collaboration with Kenji Watanabe and Takashi Taniguchi from the National Institute for Materials Science (NIMS) in Japan, noted they have successfully improved the junction characteristics between graphene and superconducting electrodes. 

When two sheets of are stacked together and offset at a slight angle, the bilayer material can produce numerous intriguing effects, notably superconductivity. Cornell University researchers have gained new understanding on how twisted bilayer graphene achieves this state, by identifying its highest achievable superconducting temperature – 60 Kelvin. The finding is said to be mathematically exact, a rare feat in the field, and is spurring new insights into the factors that fundamentally control superconductivity. 

“Looking ahead, this paves the way for understanding what are the possible degrees of freedom that one should try to control and optimize in order to enhance the tendency towards superconductivity in these two-dimensional material platforms,” said Debanjan Chowdhury, who co-authored the recent study.