Exotic New Superconductors Delight and Confound
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Welcome to the Quanta Science Podcast. Each episode we bring you stories about developments in science and mathematics. I'm Susan Vallett. Three new species of superconductivity were spotted last year, illustrating the many ways electrons can join together to form a frictionless quantum soup. That's next. Quantum Magazine is an editorially independent online publication supported by the Simons Foundation to enhance public understanding of science.

Superconductivity, or the flow of electric current with zero resistance, was discovered in three distinct materials last year.

Two instances stretch the textbook understanding of the phenomenon. The third shreds it completely. Ashwin Vishwanath is a physicist at Harvard University who wasn't involved in the discoveries. He says if you take the picture that experiments seem to be pointing to, it's an extremely unusual form of superconductivity that a lot of people would have said is not possible.

Ever since 1911, when Dutch scientist Heike Kamerlingh Onnes first saw electrical resistance vanish, superconductivity has captivated physicists. There's the pure mystery of how it happens. The phenomenon requires electrons, which carry electrical current, to pair up. Electrons repel each other, so how can they be united? Then there's the technological promise.

Already, superconductivity has enabled the development of MRI machines and powerful particle colliders. If physicists could fully understand how and when the phenomenon arises, perhaps they could engineer a wire that superconducts electricity under everyday conditions rather than exclusively at low temperatures, as currently the case. World-altering technologies like lossless power grids and magnetically levitating vehicles might follow.

The recent spate of discoveries has both compounded the mystery of superconductivity and heightened the optimism. Matthew Yankowitz is a physicist at the University of Washington. It seems to be that in materials, like superconductivity is everywhere.

A lot of materials superconduct. It's not that rare to find superconductivity. The discoveries stem from a recent revolution in material science. All three new instances of superconductivity arise in devices assembled from flat sheets of atoms.

These materials display unprecedented flexibility. At the touch of a button, physicists can switch them between conducting, insulating, and more exotic behaviors, a modern form of alchemy that has supercharged the hunt for superconductivity. It now seems increasingly likely that diverse causes can give rise to the phenomenon.

Just as birds, bees, and dragonflies all fly using different wing structures, materials seem to pair electrons together in different ways. Even as researchers debate exactly what's happening in the various two-dimensional materials in question, they anticipate that the growing zoo of superconductors will help them achieve a more universal view of the alluring phenomenon.

The case of Kamerlingh Onnes's observations and superconductivity seen in other extremely cold metals was finally cracked in 1957.

John Bardeen, Leon Cooper, and John Robert Schrieffer figured out that at low temperatures, a material's jittery atomic lattice quiets down, so more delicate effects come through. Electrons gently tug on protons in the lattice, drawing them inward to create an excess of positive charge. That deformation is known as a phonon. It can then draw in a second electron, forming a Cooper pair.

These Cooper pairs of electrons can all come together into a coherent quantum entity in a way that lone electrons can't. The resulting quantum soup slips frictionlessly in between the material's atoms, which normally impede electric flow. Bardeen, Cooper, and Schrieffer's theory of phonon-based superconductivity earned them the Physics Nobel Prize in 1972. But it turned out not to be the whole story.

In the 1980s, physicists found that Cooper-filled crystals called cuprates could superconduct at higher temperatures, where atomic jiggles wash out phonons. Other similar examples followed. So theorists brainstormed new ways of pairing electrons. The higher temperature superconductors seem to have atoms arranged in a way that slows electrons down.

and when electrons get the chance to mingle in a leisurely fashion, they collectively generate an ornate electric field that can make them do novel things, like form pairs rather than repel. Physicists now suspect that in cuprates specifically, electrons hop between atoms in a particular way that favors pairing. But other unconventional superconductors are still quite mysterious.

Then, in 2018, a new superconductor opened physicists' eyes wider. Pablo Hario Herrero, a physicist at MIT, found that if you took a sheet of carbon atoms arranged in a honeycomb lattice, a 2D crystal called graphene, and twisted it precisely at 1.1 degrees and stacked it on top of another graphene sheet, the two layers could superconduct.

Researchers had already been dabbling with 2D materials and finding diverse behaviors. By applying electric fields, they could add electrons to the sheet, or make the electrons feel almost as if the atomic grid were contracting. Twiddling these settings in a single 2D device could reproduce the behavior of thousands to millions of potential materials. Hario Herrero showed that among those heaps of possibilities was a new superconductor: magic angle graphene.

Then, a couple of years later, a group in California removed the magic angle, finding that three-layer, twist-free graphene devices could also superconduct. Researchers are still discussing why electrons stick together in these cases. Phonons fit the data in some ways, but something new also seems responsible. But what really thrilled physicists was the promise of a fresh way to investigate superconductivity in general.

The customizable 2D devices had freed them from the drudgery of designing, growing, and testing new crystals one by one.

Researchers would now be able to quickly recreate the effects of many different atomic lattices in a single device and find out exactly what electrons are capable of. The research strategy is paying off. In 2024, physicists found the first instances of superconductivity in 2D materials other than graphene, along with a completely novel form of superconductivity in a new graphene system.

The discoveries have established that the earlier graphene superconductors marked just the outskirts of a new wild jungle. Before that, in 2020, physicist Cory Dean and his team at Columbia University tried stacking sheets of a different 2D crystal.

This one was a honeycomb arrangement of two types of atoms called a transition metal dichalcoconide, or TMD. When they twisted the sheets at 5 degrees, the resistance plunged toward zero, but didn't stay there. It was an inconclusive hint of superconductivity.

The tentative nature of the detection didn't stop Liang Fu of MIT and Konstantin Shrada of Louisiana State University from trying to explain it. They suspected that phonons weren't the answer.

Twisted materials are powerful because the twist changes what the electrons experience, imbuing the material with a kaleidoscopic moire pattern. The moire features large hexagonal cells that act like artificial atoms, hosting electrons. In this new environment, electrons move slowly enough for their collective electrical interactions to guide their behavior.

But how were the electrons conspiring to form pairs? The Columbia group funneled electrons into the moire. They observed that when there was one electron for each of the large cells in the moire material, these electrons assumed an anti-ferromagnetic arrangement, meaning their intrinsic magnetic fields tended to alternate between pointing up and down.

adding extra electrons to the moire made the resistance drop to zero. Cooper pairs had formed. Fu and Trada argued that the same electron-on-electron action was making both the anti-ferromagnetic state and the superconducting state possible.

At one electron per cell, each electron can have a preferred location and magnetic orientation. But when additional electrons pile on, the magnetic arrangement becomes unstable, and the whole population starts to flow freely. Scientific journals initially rejected Fu and Trotta's paper describing these ideas because there wasn't any hard evidence that TMDs can superconduct. Now there is.

The Columbia group spent four years improving their ability to measure electrical resistance at low temperatures, and last year they had a breakthrough. They assembled another two-sheet device with a five-degree twist, cooled it down, and watched it superconduct, an observation published in Nature in January. Here's physicist Corey Dean. So it's sort of a little bit of indication that like that actually was correct. Food.

Foo and Schrader's theory, bolstered by the Columbia confirmation, has been published, but it isn't proved. One way to test it is to check whether the Cooper pairs can rotate, as the theory predicts. That's an unusual feature, as electrons paired by phonons don't orbit each other. Adding electrons to an antiferromagnetic metal isn't the only way to cook up superconductivity in TMDs.

Shortly before the Columbia discovery, another group found an even more peculiar species of superconductivity in the very same material. Ji Shan and Kin-Fai Mack, an academic power couple who run a lab at Cornell University, had been searching for superconductivity in TMDs since Hario Herrero's blockbuster twisted graphene discovery in 2018.

They spent years mixing and matching five kinds of TMD crystals, trying out different twist angles and temperatures, and applying various electric field strengths to the material. They were searching a massive haystack for a superconducting device. When the needle finally appeared, it displayed a species of superconductivity that no one had seen coming.

The Columbia team had started with an antiferromagnetic metal and added electrons. The Cornell group started with an insulator and added nothing. Their moire pattern resulted from a milder 3.5 degree twist. It allowed electrons to slow down so much and interact so strongly that they all got stuck in place at precisely one electron per cell.

Then the group made the device superconduct just by tweaking the strength of the applied electric field. That result, which the researchers reported in Nature in October of 2024, doesn't neatly fit any popular theory of superconductivity. Vishwanath says it really smells like something else is going on.

Even as superconductivity has spread to the TMDs, graphene continues to astonish. Over the summer, a graphene device produced a mythical form of superconductivity. Longzhu, leader of the group that found it, says it's phenomenologically different from all other superconductors.

Twisting is too messy for Ju's taste. The moire patterns tend to get disrupted by wrinkles in the sheets that make every device a little different. Instead, he studies a staircase-like arrangement of four graphene layers that can also slow electrons down. The challenge is to spot which graphene flakes naturally have this staircase arrangement. That's something Ju accomplishes with the aid of an infrared camera.

In 2023, Zhu's group made a splash when they placed a five-layer graphene flake on an insulator at a twisted angle and observed a rare electron behavior that normally requires a strong magnetic field to induce. The theorist questioned whether the twist was essential, so he and his team went back to see what would happen when they took the twist out. He says they found something that was even more bizarre.

As they changed the strength of the electric field that they applied to the material, they found several settings where resistance vanished. In two cases, the superconductivity flickered, with resistance coming and going. Strangely, when they switched on a nearby magnet, the flickering stopped. Magnets normally kill superconductivity, but here they strengthened it.

Jue's group suspects that their graphene staircases are creating the conditions for electrons to pair up and rotate. But they think that in their graphene devices, all pairs tend to rotate in the same direction, either clockwise or counterclockwise, and flickers appear when pairs aren't all rotating uniformly. The magnetic field stamps out the flickers by pushing any wayward pairs to align with the overall gyre.

A material with such a preferred internal direction is called a chiral, but chirality has long been thought to preclude superconductivity, since it distinguishes leftward and rightward moving electrons in a way that should stop pairs from forming. It's so unusual that other researchers are waiting for more experiments to verify it. Here's Mac. I would say that, you know, it's probably still an evolving story at this point. I think

I think it just needs additional data to fully confirm whether it's a chiral superconductor or not. And if it's confirmed, it's actually a very exciting discovery, right? It's definitely an unconventional superconductor. Theorists, meanwhile, have published new theories of how chiral superconductivity might happen. Fu and collaborators proposed the following recipe last fall.

You start with electrons arranged to form a repeating crystal, like in an insulator, except in this case, the electron grid is free to float independently of the background atomic nuclei. Then, the electron grid relaxes, and its ripples pair electrons the way phonons do. Foo stresses that this is just one possibility, noting that we're in uncharted territory.

While physicists can't say for sure what's pairing electrons in these 2D materials, they feel more confident that there are multiple ways to do it. Electrons organize into all sorts of materials, from insulators to magnetic metals to electronic crystals, and slight disturbances seem poised to tip many of these materials into superconducting electron pairs.

Being able to directly see what happens when they add more electrons to a material or slightly weaken its electric field lets physicists quickly try out an unprecedented number of recipes and see which ones lead to superconductivity. Here's Dean again. The real promise of the twisted bilayer systems is this is really a tunable lab in which we can make basically any material. The experimentalists are amassing a treasure trove of data for theorists to explain.

Mack and Shan hope that this abundance will let theorists predict ways to create superconductivity that experiments can confirm. That would demonstrate a true understanding of the phenomenon. It would mark both an academic achievement and a key step toward designing materials for revolutionary new technologies.

But for now, experimentalists are still the ones leading the way. Or in the words of physicist Matthew Yankovic, Everyone's like rushing as fast as they can to do this still. I can't believe that we're six years in and like there's no, you can't take a break.

Arlene Santana helped with this episode. I'm Susan Vallett. For more on this story, read Charlie Wood's full article, Exotic New Superconductors Delight and Confound, on our website, quantamagazine.org. Make sure to tell your friends about the Quanta Science Podcast and give us a positive review or follow where you listen. It helps people find this podcast.