MIT

Researchers at MIT, University of Basel, Florida State University, the University of Florida and the National Institute for Materials Science in Japan have reported a new family of unconventional superconducting states in rhombohedral multilayer graphene, revealing magnetic-field-enhanced superconductivity that persists far beyond conventional limits.

The work focuses on rhombohedral graphene - naturally occurring stacks of four or five graphene layers arranged in a staircase-like configuration within graphite. Unlike engineered twisted structures, this system combines intrinsic structural order with exceptionally flat electronic bands, enabling strong electronic correlations that can be tuned via electrostatic gating. By exfoliating graphite and isolating these rare stacking domains, the team accessed ultraclean samples suitable for probing delicate superconducting phases.

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.

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.

An international team of researchers, including ones from ICFO, MIT, RWTH Aachen University and others, has used terahertz light to explore exotic phenomena within magic-angle twisted bilayer graphene. This approach revealed previously unseen behaviors and provided direct insights into the quantum geometry of electronic wavefunctions —the fundamental framework underlying these phenomena.

It has been less than ten years since scientists placed two graphene layers on top of each other, twisted them exactly 1.1º and observed the emergence of exotic phenomena like superconductivity and topological phases of matter. The unlocked new physics attracted great attention among the community, and the whole system soon became known as “magic-angle twisted bilayer graphene”. This magic-angle has been extensively studied ever since with most efforts focused on understanding how electron interactions lead to such exotic collective quantum phases. However, electrons at the single-particle level have also been predicted to exhibit intriguing quantum behaviors underscored by the geometry of their wavefunctions – their quantum geometry. Yet, observing these behaviors has proven challenging.

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 from MIT, Princeton University, SLAC National Accelerator Laboratory and Japan's National Institute for Materials Science have created a new ultrathin 2D material with unusual magnetic properties that initially surprised the researchers before they went on to solve the complicated puzzle behind those properties’ emergence. As a result, the work introduces a new platform for studying how materials behave at the most fundamental level — the world of quantum physics.

The scientists, led by MIT's Pablo Jarillo-Herrero, worked with three layers of graphene. Each layer was twisted on top of the next at the same angle, creating a helical structure reminiscent of a DNA helix.

MIT researchers have gained new understanding of what leads electrons to split into fractions of themselves. Their solution sheds light on the conditions that give rise to exotic electronic states in graphene and other two-dimensional systems.

The recent work attempts to make sense of a discovery that was reported earlier this year by a different group of physicists at MIT, led by Assistant Professor Long Ju. Ju’s team found that electrons appear to exhibit “fractional charge” in pentalayer graphene — a configuration of five graphene layers that are stacked atop a similarly structured sheet of boron nitride.

Researchers at the Massachusetts Institute of Technology (MIT), University of Texas at Dallas and Japan's National Institute for Materials Science have reported the quantum anomalous Hall effect (QAHE), a topological phenomenon that features quantized Hall resistance at zero magnetic field, in a rhombohedral pentalayer graphene-monolayer tungsten disulfide (WS2) heterostructure. 

This achievement can also be described as a 'five-lane superhighway' for electrons, that could allow ultra-efficient electronics and more. The team explained that its discovery could have direct implications for low-power electronic devices because no energy is lost during the propagation of electrons, which is not the case in regular materials where the electrons are scattered.

Researchers from the University of Göttingen, Japan's National Institute for Materials Science and Massachusetts Institute of Technology (MIT) have demonstrated experimentally that electrons in naturally occurring double-layer graphene move like particles without any mass, in the same way that light travels. Furthermore, they have shown that the current can be "switched" on and off, which has potential for developing tiny, energy-efficient transistors. 

Among its many unusual properties, graphene is known for its extraordinarily high electrical conductivity due to the high and constant velocity of electrons travelling through this material. This unique feature has made scientists try to use graphene for faster and more energy-efficient transistors. The challenge has been that to make a transistor, the material needs to be controlled to have a highly insulating state in addition to its highly conductive state. In graphene, however, such a "switch" in the speed of the carrier cannot be easily achieved. In fact, graphene usually has no insulating state, which has limited graphene's potential a transistor.