Superconductors are like the express trains in a metro system. Any electricity that "boards" a superconducting material can zip through it without stopping and losing energy along the way. As such, superconductors are extremely energy efficient, and are used today to power a variety of applications, from MRI machines to particle accelerators.
But these "conventional" superconductors are somewhat limited in terms of uses because they must be brought down to ultra-low temperatures using elaborate cooling systems to keep them in their superconducting state. If superconductors could work at higher, room-like temperatures, they would enable a new world of technologies, from zero-energy-loss power cables and electricity grids to practical quantum computing systems. And so scientists at MIT and elsewhere are studying "unconventional" superconductors - materials that exhibit superconductivity in ways that are different from, and potentially more promising than, today's superconductors.
In a promising breakthrough, MIT physicists have today reported their observation of new 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, reported in the journal Science, offers the most direct confirmation yet that the material exhibits unconventional superconductivity.
In particular, 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.
"There are many different mechanisms that can lead to superconductivity in materials," says study co-lead author Shuwen Sun, a graduate student in MIT's Department of Physics. "The superconducting gap gives us a clue to what kind of mechanism can lead to things like room-temperature superconductors that will eventually benefit human society."
The researchers made their discovery using a new experimental platform that allows them to essentially "watch" the superconducting gap, as the superconductivity emerges in two-dimensional materials, in real-time. They plan to apply the platform to further probe MATTG, and to map the superconducting gap in other 2D materials - an effort that could reveal promising candidates for future technologies.
"Understanding one unconventional superconductor very well may trigger our understanding of the rest," says Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at MIT and the senior author of the study. "This understanding may guide the design of superconductors that work at room temperature, for example, which is sort of the Holy Grail of the entire field."
The study's other co-lead author is Jeong Min Park PhD '24; Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Japan are also co-authors.
The ties that bind
Graphene is a material that comprises a single layer of carbon atoms that are linked in a hexagonal pattern resembling chicken wire. A sheet of graphene can be isolated by carefully exfoliating an atom-thin flake from a block of graphite (the same stuff of pencil lead). In the 2010s, theorists predicted that if two graphene layers were stacked at a very special angle, the resulting structure should be capable of exotic electronic behavior.
In 2018, Jarillo-Herrero and his colleagues became the first to produce magic-angle graphene in experiments , and to observe some of its extraordinary properties. That discovery sprouted an entire new field known as "twistronics," and the study of atomically thin, precisely twisted materials. Jarillo-Herrero's group has since studied other configurations of magic-angle graphene with two, three , and more layers , as well as stacked and twisted structures of other two-dimensional materials. Their work, along with other groups, have revealed some signatures of unconventional superconductivity in some structures.
Superconductivity is a state that a material can exhibit under certain conditions (usually at very low temperatures). When a material is a superconductor, any electrons that pass through can pair up, rather than repelling and scattering away. When they couple up in what is known as "Cooper pairs," the electrons can glide through a material without friction, instead of knocking against each other and flying away as lost energy. This pairing up of electrons is what enables superconductivity, though the way in which they are bound can vary.
"In conventional superconductors, the electrons in these pairs are very far away from each other, and weakly bound," says Park. "But in magic-angle graphene, we could already see signatures that these pairs are very tightly bound, almost like a molecule. There were hints that there is something very different about this material."
Tunneling through
In their new study, Jarillo-Herrero and his colleagues aimed to directly observe and confirm unconventional superconductivity in a magic-angle graphene structure. To do so, they would have to measure the material's superconducting gap.
"When a material becomes superconducting, electrons move together as pairs rather than individually, and there's an energy 'gap' that reflects how they're bound," Park explains. "The shape and symmetry of that gap tells us the underlying nature of the superconductivity."
Scientists have measured the superconducting gap in materials using specialized techniques, such as tunneling spectroscopy. The technique takes advantage of a quantum mechanical property known as "tunneling." At the quantum scale, an electron behaves not just as a particle, but also as a wave; as such, its wave-like properties enable an electron to travel, or "tunnel," through a material, as if it could move through walls.
Such tunneling spectroscopy measurements can give an idea of how easy it is for an electron to tunnel into a material, and in some sense, how tightly packed and bound the electrons in the material are. When performed in a superconducting state, it can reflect the properties of the superconducting gap. However, tunneling spectroscopy alone cannot always tell whether the material is, in fact, in a superconducting state. Directly linking a tunneling signal to a genuine superconducting gap is both essential and experimentally challenging.
In their new work, Park and her colleagues developed an experimental platform that combines electron tunneling with electrical transport - a technique that is used to gauge a material's superconductivity, by sending current through and continuously measuring its electrical resistance (zero resistance signals that a material is in a superconducting state).
The team applied the new platform to measure the superconducting gap in MATTG. By combining tunneling and transport measurements in the same device, they could unambiguously identify the superconducting tunneling gap, one that appeared only when the material exhibited zero electrical resistance, which is the hallmark of superconductivity. They then tracked how this gap evolved under varying temperature and magnetic fields. Remarkably, the gap displayed a distinct V-shaped profile, which was clearly different from the flat and uniform shape of conventional superconductors.
This V shape reflects a certain unconventional mechanism by which electrons in MATTG pair up to superconduct. Exactly what that mechanism is remains unknown. But the fact that the shape of the superconducting gap in MATTG stands out from that of the typical superconductor provides key evidence that the material is an unconventional superconductor.
In conventional superconductors, electrons pair up through vibrations of the surrounding atomic lattice, which effectively jostle the particles together. But Park suspects that a different mechanism could be at work in MATTG.
"In this magic-angle graphene system, there are theories explaining that the pairing likely arises from strong electronic interactions rather than lattice vibrations," she posits. "That means electrons themselves help each other pair up, forming a superconducting state with special symmetry."
Going forward, the team will test other two-dimensional twisted structures and materials using the new experimental platform.
"This allows us to both identify and study the underlying electronic structures of superconductivity and other quantum phases as they happen, within the same sample," Park says. "This direct view can reveal how electrons pair and compete with other states, paving the way to design and control new superconductors and quantum materials that could one day power more efficient technologies or quantum computers."
This research was supported, in part, by the U.S. Army Research Office, the U.S. Air Force Office of Scientific Research, the MIT/MTL Samsung Semiconductor Research Fund, the Sagol WIS-MIT Bridge Program, the National Science Foundation, the Gordon and Betty Moore Foundation, and the Ramon Areces Foundation.
