New Method Dials Up Superconductivity

Two electrons shouldn’t want anything to do with each other. They carry the same charge, so they repel, always have, always will. And yet in superconductors they do something altogether stranger: they pair up, locking into duets that glide through a material without losing any energy to resistance. For decades, physicists assumed those pairings were brokered by vibrations in the atomic lattice, tiny phonons nudging reluctant electrons together the way a mutual friend might coax two stubborn people into conversation. But new experiments on twisted bilayer graphene suggest the matchmaker might actually be the electrons’ own repulsive force, and that weakening it doesn’t help. It kills the superconductivity entirely.

The finding, published in Nature Physics on 7 April, comes from a team led by Chun Ning (Jeanie) Lau at Ohio State University. It has implications that stretch well beyond a single exotic material, potentially reshaping how physicists think about engineering superconductors from the ground up.

Twisted bilayer graphene is, in principle, a deceptively simple thing. You take two atom-thick sheets of carbon, stack them, and rotate one by roughly 1.05 degrees (the so-called “magic angle”). At that tiny twist the material’s electronic bands flatten dramatically, electrons slow to a crawl, and correlations between them explode. Superconductivity appears. Insulating states appear. A whole zoo of quantum phases emerges from what is, structurally, little more than two layers of pencil lead. The trouble is that nobody has been able to pin down why the superconductivity forms in the first place. Phonons? Electron-electron interactions? Some unholy combination? The debate has raged since 2018, when the first magic-angle devices were reported.

Lau’s team took a different tack. Rather than building more devices and hoping to stumble across the answer, they built a system where they could dial the strength of electron interactions up and down, in situ, and watch what happened.

The trick was strontium titanate, or STO, a synthetic crystal whose dielectric constant is enormous and, crucially, tunable. At low temperatures STO’s dielectric constant can reach 25,000, roughly 6,000 times that of silicon dioxide. By placing their twisted bilayer graphene on an STO substrate with only a thin (3 to 4 nanometre) spacer of hexagonal boron nitride in between, the researchers could use the substrate’s electric polarisation to screen the Coulomb interactions between electrons in the graphene. Crank up the dielectric constant and you muffle the repulsive chatter between electrons. Dial it back and you let them shout at each other again.

“Electrons normally repel each other, but in superconductors they form pairs; this pair formation is the key to a superconductor’s ability to conduct electricity without dissipation,” Lau said. “Our evidence suggests that electrons themselves, depending on their sensitivity to their nearby environment, are unexpectedly important for material changes.”

What they saw was striking, and counterintuitive. In a conventional superconductor, damping down electron repulsion should, if anything, help. Pairs form more easily when the thing keeping them apart gets weaker. But in the magic-angle device (twist angle 1.05 degrees), increasing the dielectric constant steadily shrank the superconducting dome, the region of temperatures and carrier densities where superconductivity lives. The critical temperature dropped by a factor of four. The critical magnetic field fell by a factor of ten. Push the screening far enough and superconductivity vanished altogether. Gone. The entire dome, snuffed out. This is roughly the opposite of what you’d expect if phonons alone were doing the heavy lifting, and it points strongly toward a pairing mechanism in which the electrons’ own interactions, mediated by collective excitations like plasmons and electron-hole pairs, are essential. Suppress those interactions too aggressively and you lose the glue.

Then came the second surprise. The team built a device twisted to 1.4 degrees, well away from the magic angle, where the electronic bands are broader and interactions weaker. On a conventional substrate, such devices show neither superconductivity nor correlated insulating states. But on STO, something unexpected turned up: superconductivity appeared, while the insulating states did not. It seems the screening preferentially knocked out the competing insulating phases, clearing the way for superconductivity to emerge in a regime where it had never been seen before.

It’s a sort of double-edged sword, as the researchers put it. At the magic angle, screening weakens the interactions that drive pairing and superconductivity dies. At larger angles, screening kills the insulating states that were blocking superconductivity, and it springs to life. The same knob, turned the same direction, produces opposite outcomes depending on which phase is more vulnerable. That kind of competition between ordered states is something theorists have long suspected, but seeing it controlled so cleanly in a single experimental platform is new.

Francisco Guinea, a theorist at Imdea Nanoscience in Madrid and a co-author on the study, developed a model that captures these trends qualitatively. In the model, pairing arises from Coulomb interactions that are screened by plasmons, electron-hole pairs, and longitudinal acoustic phonons. The model predicts that as the external dielectric constant climbs, the critical temperature should fall toward zero, which is broadly what the experiments show. It doesn’t get all the numbers right (the predicted critical temperature for the large-angle device, about 25 millikelvin, is low), but the direction is consistent. And the model uses only parameters that have been well established by decades of graphene and graphite research, which gives it a sort of credibility that more exotic proposals sometimes lack.

There are caveats, naturally. Phonon-based pairing can’t be fully ruled out; it could be contributing alongside electronic interactions, and screening might affect phonon coupling too. The theoretical model, the authors are careful to say, is only an initial step. Still, the sheer scale of the superconductivity suppression (a fourfold drop in critical temperature, a tenfold drop in critical field) is hard to wave away. Previous attempts to screen twisted bilayer graphene used metallic gates placed 7 to 12 nanometres away, and superconductivity barely budged. The STO approach, with its massive dielectric constant just 3 nanometres from the graphene, finally crosses the threshold where screening bites.

Perhaps the most tantalising prospect is what comes next. If electron interactions really are central to pairing in these materials, then controlling the dielectric environment could become a general tool for engineering superconductivity across the growing family of moiré systems (twisted trilayer graphene, moiré transition-metal dichalcogenides, rhombohedral few-layer graphene). “We’re showing capabilities that we haven’t shown before, so many people in the field are getting really excited about this result,” Lau said. Whether that excitement translates into superconductors that work above the frigid temperatures of a dilution fridge is another matter entirely, but the principle, that the electromagnetic surroundings of a material can switch its quantum ground state on and off, opens a door that wasn’t there before.

DOI: 10.1038/s41567-026-03243-1