Layered cuprate crystals resemble a stack of pancakes: each pancake is a plane of copper and oxygen atoms, with spacer layers of other elements in between. Usually, superconductivity is studied in the whole stack, where layers influence each other. But what happens if you leave just a single pancake?

Angle-resolved photoemission spectroscopy, a technique that probes the electronic structure of materials, revealed that an isolated CuO₂ plane exhibits the same gap as a bulk sample. The gap is an energy window that electrons must overcome to conduct without loss. It closes at a slightly higher temperature than in the bulk material, but its shape is the same — d-wave, like a four-leaf clover.

The result implies that superconductivity in cuprates is a two-dimensional phenomenon. Neighboring layers are not required; they only slightly tune the transition temperature. Now scientists have a platform to study superconductivity in its pure form, free from the interference of the third dimension.