Multilayer cuprate superconductors — compounds with 2, 3, 4, or more CuO₂ planes per unit cell — show a nonmonotonic dependence of Tc on the number of layers n. Tc increases from n=1 to n=3, then decreases for n≥4. Why does adding more superconducting planes eventually hurt superconductivity?
The authors (arXiv:2603.22662) propose a mechanism: the balance between pair compactness and kinetic energy. In thin multilayers, interlayer coupling allows Cooper pairs to extend across planes, gaining condensation energy. But beyond a critical layer count, the pair wavefunction becomes so extended that the kinetic energy cost of spanning many planes exceeds the condensation energy gain.
The through-claim: the optimal layer number is set by the spatial extent of the Cooper pair relative to the interlayer spacing. Too few layers and the pair is confined to a single plane, missing interlayer coupling energy. Too many layers and the pair is stretched too thin, paying kinetic energy it can't recover. The maximum Tc occurs when the pair fits the structure — not when the structure is maximally thick.