The Absent Parts

Take two materials, neither superconducting — or at best weakly superconducting in bulk. Stack them as a bilayer heterostructure. The result can be substantially superconducting, with a critical temperature that neither component possesses alone.

Ummarino and Zaccone (arXiv:2603.25648, March 2026) show that this emergence arises from the combination of quantum confinement and proximity effects at the interface. Quantum confinement in the thin layer modifies the electronic density of states — creating van Hove singularities at energies that depend on the layer thickness. The proximity effect couples the two layers so that enhanced pairing in one layer leaks into the other. When the confinement-induced peak in the density of states aligns with the phonon-mediated pairing energy, the bilayer develops a Tc that can exceed both bulk values substantially.

The prediction is specific: given two materials and their electron-phonon coupling parameters, the model identifies the optimal layer thicknesses for maximum Tc enhancement. The enhancement is not a generic consequence of layering — it requires the right thickness to position the confinement-induced density of states peak at the right energy. Too thick and the confinement effect vanishes (bulk behavior). Too thin and the electronic structure changes qualitatively. The sweet spot is a few nanometers.

The structural observation: the absence of a property in the parts does not imply its absence in the whole when the combination creates new physics that the parts individually cannot access. Quantum confinement and proximity effects are interface phenomena — they exist only at the boundary between two materials and have no analogue in either material alone. The superconductivity is not hidden in the constituents waiting to be released. It is created by the geometry of their combination. The bilayer is not a sum; it is a new system whose properties depend on the interface rather than the bulk.