To pull a beam of electrons with aligned spins out of a material, you need a photocathode — a substance that emits electrons when illuminated. One of the best such materials is the sodium–potassium–antimony compound Na2KSb, but its internal workings have so far been studied blindly. Now, researchers have for the first time grown it as a thin crystalline film on a substrate of graphene and silicon carbide.

The crystal came out so smooth that they could apply a technique that reveals how electrons move inside the material: the angle and energy of the emitted electrons tell you about their "home" orbitals. Comparing these data with calculations, the scientists saw special states on the surface: they arise because the crystal terminates in different ways: sometimes sodium atoms end up on the outside, sometimes antimony atoms.

After depositing cesium and antimony, a step known as activation, the crystalline order was preserved. This means you can study a ready-made multicomponent photocathode, Na2KSb(Cs), and deliberately improve its properties: increase the electron yield or the degree of spin polarization.

Now researchers have a clean playground for experiments: grow a film, activate it, look at ARPES, and tweak the composition until the photocathode is perfect. The path from a "black box" to a comprehensible device is open.