Two paper-thin sheets of a semiconductor are stacked and slightly rotated — by about three to four degrees. The atomic lattices don't match, creating a large-scale pattern — a moiré, like the ripples from two washboards shifted by a small angle. In this pattern, electrons move quite differently than in a regular crystal.
Recently, in such a system made of tungsten selenide (WSe₂), superconductivity was discovered — a state where current flows without resistance. But it behaved capriciously: it appeared only when each moiré site held exactly one electron (half filling), and vanished instantly if a tiny bit of charge was added or removed. The authors of the new study built a model explaining this caprice. They reduced the complex moiré landscape to an effective lattice, where each site is like a little house for an electron, and neighboring houses are connected by corridors of varying lengths. It turned out that moderate repulsion between electrons prevents them from roaming freely but doesn't lock them in place — some electrons still travel in pairs, creating a superconducting condensate.
Calculations showed that this superconductivity is "gossamer": it exists only in a narrow corridor of parameters where Coulomb repulsion is partially suppressed and mobile pairs are still plentiful. Change the twist angle or electron concentration, and the web tears. That's exactly what experimentalists see: as the layers are rotated, the system smoothly transitions from an insulator (all electrons locked in their houses) to a superconductor, and then to a normal metal.
The moiré pattern in twisted layers can be hundreds of times larger than the interatomic distance — roughly like fitting a city map on your palm with houses a couple of centimeters across.
In Robert Wilson's novel "Spin," characters manipulate the properties of spacetime using thin films — a distant echo of how twisting layers changes the rules for electrons.