Can the noble metals (Au, Ag, and Cu) be superconductors?

It is common knowledge that noble metals are excellent conductors but do not exhibit superconductivity. On the other hand, quantum confinement in thin films has been consistently shown to induce a significant enhancement of the superconducting critical temperature in several superconductors. It is, therefore, an important fundamental question whether ultrathin film confinement may induce observable superconductivity in nonsuperconducting metals. We present a generalization, in the Eliashberg framework, of a BCS theory of superconductivity in good metals under thinfilm confinement. By numerically solving these new Eliashberg-type equations, we find the dependence of the superconducting critical temperature on the film thickness L. This parameter-free theory predicts a maximum increase in the critical temperature for a specific value of the film thickness, which is a function of the number of free carriers in the material. Exploiting this fact, we predict that ultrathin films of gold, silver, and copper of suitable thickness could be superconductors at low but experimentally accessible temperatures. We demonstrate that this is a fine-tuning problem where the thickness must assume a very precise value, close to half a nanometer. Finally we applied the same method to magnesium and found a significantly higher critical temperature than that of noble metals.