PHYSICISTS ARE BEWITCHED BY TWISTED GRAPHENE’S ‘MAGIC ANGLE’
PABLO JARILLO-HERRERO IS channeling some of his copious energy into a morning run, dodging startled pedestrians as he zips along, gradually disappearing into the distance. He’d doubtlessly be moving even faster if he weren’t dressed in a sports coat, slacks, and dress shoes, and confined to one of the many weirdly long corridors that crisscross
In the summer of 2017, doctoral student Yuan Cao, who at the age of 21 was already in his third year of graduate school at MIT, brought Jarillo-Herrero a new device that gave him reason to pay attention. As before, an electric field switched the device into an insulator. But this time they tried cranking up the field higher, and it suddenly switched again—into a superconductor.
The lab spent the next six months duplicating the results and nailing down measurements. The work was done in strict secrecy, a break from the typically highly open and collaborative culture of the twisted bilayer graphene field. “I had no way of knowing who else might be close to superconductivity,” said Jarillo-Herrero. “We share ideas and data all the time in this field, but we’re also very competitive.”
In January 2018, with a paper prepared, he called an editor at Nature, explained what he had, and made his submission contingent on the journal agreeing to a one-week review process—a friend had told him one of the seminal CRISPR papers had received that extraordinary treatment. The journal agreed, and the paper flew through the rush review.
Jarillo-Herrero sent a prepublication email heads-up to MacDonald, who hadn’t even known that Jarillo-Herrero had been doggedly pursuing the magic angle. “I couldn’t believe it,” said MacDonald. “I mean I actually found it beyond belief.” Dean learned about it along with the rest of the physics community at a conference in March 2018, right around the time that the Nature paper came out. “The results proved me spectacularly wrong,” Dean said.
The Perfect Playground
Physicists are excited about magic-angle twisted bilayer graphene not because it’s likely to be a practical superconductor but because they’re convinced it can illuminate the mysterious properties of superconductivity itself. For one thing, the material seems to act suspiciously like a cuprate, a type of exotic ceramic in which superconductivity can occur at temperatures up to about 140 kelvin, or halfway between absolute zero and room temperature. In addition, the sudden jumps in twisted bilayer graphene—from conducting to insulating to superconducting—with just a tweak of an external electric field indicate that free electrons are slowing to a virtual halt, notes physicist Dmitri Efetov of the Institute of Photonic Sciences (ICFO) in Barcelona, Spain. “When they stop, [the electrons] interact all the more strongly,” he said. “Then they can pair up and form a superfluid.” That fluidlike electron state is considered a core feature of all superconductors.
The main reason 30 years of studying cuprates has shed relatively little light on the phenomenon is that cuprates are complex, multi-element crystals. “They’re poorly understood materials,” said Efetov, noting that they superconduct only when precisely doped with impurities during their demanding fabrication in order to add free electrons. Twisted bilayer graphene, on the other hand, is nothing but carbon, and “doping” it with more electrons merely requires applying a readily varied electric field. “If there’s any system where we can hope to understand strongly correlated electrons, it’s this one,” said Jarillo-Herrero. “Instead of having to grow different crystals, we just turn a voltage knob, or apply more pressure with the stamps, or change the rotation angle.” A student can try to change the doping in an hour at virtually no cost, he notes, versus the months and tens of thousands of dollars it might take to try out a slightly different doping scheme on a cuprate.
Also unique, said MacDonald, is the small number of electrons that seem to be doing the heavy lifting in magic-angle twisted bilayer graphene—about one for every 100,000 carbon atoms. “It’s unprecedented to see superconducting at such a low density of electrons,” he said. “It’s lower than anything else we’ve seen by at least an order of magnitude.” Over 100 papers have popped up on the scientific preprint server arxiv.org that offer theories to explain what might be going on in magic-angle twisted bilayer graphene. Andrei Bernevig, a theoretical physicist at Princeton University, calls it “a perfect playground” for exploring correlated physics.
Physicists seem eager to play on it. Besides being able to flip between extremes in conductivity with a literal push of a button, notes Rebeca Ribeiro-Palau, a physicist at the Center for Nanoscience and Nanotechnology near Paris, there’s already good evidence that twisted bilayer graphene’s magnetic, thermal and optical properties can be nudged into exotic behaviors as easily as its electronic properties can. “In principle you can switch any property of matter on and off,” she said. MacDonald points out, for example, that some of the insulating states in twisted bilayer graphene appear to be accompanied by magnetism that arises not from the quantum spin states of the electrons, as is typically the case, but entirely from their orbital angular momentum—a theorized but never-before-observed type of magnetism.
The Coming Age of Twistronics
Now that Jarillo-Herrero’s group has proven that magic angles are a thing, physicists are trying to apply the twistronics approach to other configurations of graphene. Kim’s group has been experimenting with twisting two double-layers of graphene and has already found evidence of superconductivity and correlated physics. Others are stacking up three or more layers of graphene in the hopes of gaining superconductivity at other magic angles, or perhaps even when they are aligned. Bernevig posits that as the layers stack up higher and higher, physicists may be able to get the superconductivity temperature to climb along with it. Other magic angles may play a role, too. Some groups are squeezing the sheets more tightly together in order to increase the magic angle, making it easier to achieve, while MacDonald suggests even richer physics may emerge at smaller, if much harder to target, magic angles.
Meanwhile, other materials are coming into the twistronics picture. Semiconductors and transitional metals can be deposited in twisted layers and are seen as good candidates for correlated physics—perhaps better than twisted bilayer graphene. “People are thinking of hundreds of materials than can be manipulated this way,” said Efetov. “Pandora’s box has been opened.”
Dean and Efetov are among those sticking with what might already be called classic twistronics, in the hopes of boosting correlated effects in magic-angle twisted bilayer graphene devices by literally smoothing out the wrinkles in their fabrication. Because there’s no chemical bonding to speak of between the two layers, and because the slightly offset layers try to settle into alignment, forcing them to hold a magic-angle twist creates stresses that lead to submicroscopic hills, valleys and bends. Those local distortions mean that some regions of the device might be within the magic range of twist angles, while other regions are not. “I’ve tried gluing the edges of the layers, but there are still local variations,” he complained. “Now I’m trying to figure out ways to minimize the initial strain when the layers are pressed together.” Efetov has recently reported progress in doing just that, and the results have already paid off in new superconducting states at temperatures of about 3 degrees kelvin, or twice as high as previously observed.
Having burst far out into the lead of the twisted bilayer graphene field in stunning fashion, Jarillo-Herrero isn’t sitting back and waiting for others to catch up. His lab’s main focus remains trying to coax ever more exotic behavior out of twisted bilayer graphene, taking advantage of the fact that through long trial and error he’s boosted his yield of superconducting samples to nearly 50 percent. Most other groups are struggling with yields a tenth of that or less. Given that it takes about two weeks to fabricate and test a device, that’s an enormous productivity edge. “We think we’re just beginning to see all the fascinating states that will come out of these magic-angle graphene systems,” he said. “There’s a vast phase space to explore.” But to cover his bases, he’s pulled his lab into also exploring twistronics in other materials.
The stakes in the race to come up with easier to make, better performing, higher-temperature superconductors are huge. Aside from the oft-evoked vision of levitating trains, reducing the energy loss in electric power transmission would boost economies and sharply cut harmful emissions around the world. Qubit fabrication could suddenly become practical, perhaps ushering in the rise of quantum computers. Even without superconductivity, ordinary computers and other electronics could get a huge boost in performance versus cost from twistronics, due to the fact that entire complex electronic circuits could in theory be built into a few sheets of pure carbon, without needing a dozen or more complexly etched layers of challenging materials common to today’s chips. “You could integrate wildly different properties of matter into these circuits right next to one another, and vary them with local electric fields,” said Dean. “I can’t find words to describe how profound that is. I’d have to make something up. Maybe dynamic material engineering?”
However such hopes ultimately pan out, for now the excitement in twisted bilayer graphene seems only to be building. “Some may be shy to say it, but I’m not,” said Castro Neto. “If the field keeps going the way it is now, somebody is going to get a Nobel Prize out of this.” That sort of talk is probably premature, but even without it there’s plenty of pressure on Jarillo-Herrero. “What my lab did creates unrealistic expectations,” he admits. “Everyone seems to think we’re going to produce a new breakthrough every year.” He’s certainly determined to make further important contributions, he said, but he predicts that whatever the next electrifying discovery is, it’s as likely come out of a different lab as it is his. “I’ve already accepted that as a fact, and I’m fine with it,” he said. “It would be boring to be in a field where you’re the only one advancing it.”
Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.