Turning non-superconducting elements into superconductors by quantum confinement and proximity

Elemental good metals, including noble metals (Cu, Ag, Au) and severals-block elements, do not exhibit superconductivity in bulk at ambient pressure, primarily due to weak electron-phonon coupling that fails to overcome Coulomb repulsion. Quantum confinement in ultra-thin films is known to strongly reshape the electronic spectrum and the density of states (DOS) near the Fermi level, and in established superconductors it produces pronounced, often non-monotonic, thickness dependencies of the critical temperature. In this perspective, we examine whether quantum confinement alone, or in combination with proximity effects, can induce an observable superconducting instability in metals that are non-superconducting in bulk form. We review recent theoretical progress and present a unified framework based on a confinement-generalized, isotropic one-band Eliashberg theory, in which the normal DOS becomes energy dependent and key material parameters (EF, lambda and mu*) acquire an explicit thickness dependence. By numerically solving the resulting Eliashberg equations usingab-initioor experimentally determined electron-phonon spectral functions a2F and Coulomb pseudopotentials mu*, and without introducing adjustable parameters, we compute the critical temperatureTcas a function of film thickness for representative noble, alkali, and alkaline-earth metals. The theory predicts that superconductivity can emerge only in selected cases and within extremely narrow thickness windows, typically centered around sub-nanometer scales (L 0.4-0.6 nm), highlighting a pronounced fine-tuning requirement for confinement-induced superconductivity in good metals. We further discuss layered superconductor/normal-metal systems, where quantum confinement and proximity effects coexist. In these heterostructures, a substantial enhancement of the critical temperature is predicted, even when the constituent materials are non-superconducting or poor superconductors in bulk form.