Can the noble metals become superconductors?

Superconductivity is the phenomenon by which, at sufficiently low temperatures, electric current can flow in a metal with no resistance. While certain metals are excellent superconductors, other metals cannot superconduct at all.

Understanding what makes a metal superconducting is still an open question in fundamental condensed matter physics. Also, superconductivity in simple elemental solids is of great importance for technological applications—consider that elemental aluminum is used ubiquitously for superconducting Josephson junctions employed in quantum computing (e.g., for qubit implementation).

For these reasons, an important line of research in recent decades has been devoted to finding new elements that could become superconducting under certain conditions.

In this paper, published in Physical Review Materials, we proposed a completely new twist to the search for new elements that can become superconductors.

Over the past few years, I have worked on a new approach to quantitatively determine how the thickness of a thin film influences the electronic properties of a very thin metal slab. The key principle is that only electron waves will be allowed to populate the thin slab which has a wavelength that does not exceed the size of a sample along a certain propagating direction.

Clearly, along the direction orthogonal to the slab surface, the maximum allowed wavelength will be of the order of the film thickness, whereas in the perpendicular plane, the electron wave will be able to propagate with any wavelength.

A first implementation of this quantum confinement model with my former student Riccardo Travaglino yielded encouraging results for ultra-thin films of lead in comparison with experimental data in the literature.

However, the calculation was based on the Bardeen-Cooper-Schrieffer theory of superconductivity, which falls short of accuracy when the coupling between electrons and phonons in the lattice is strong and does not account in a realistic way for the phonon spectrum.

I then started collaborating with colleague Giovanni Ummarino in Turin, who is an expert of the more refined and realistic Eliashberg theory of superconductivity, which can handle all the above mentioned points in an accurate and fully quantitative way, without any adjustable parameter.

Using the most refined version of the Eliashberg theory of superconductivity combined with ab-initio simulations and with a recent quantum confinement model for quasi-2D materials, we have now predicted that noble metals become superconducting when cast into ultra-thin films of thickness lower than 1 nm.

The theory is fully quantitative and free of adjustable parameters. The most striking prediction is that ultra-thin gold (Au) films of about half a nanometer thickness become superconducting with a critical superconducting temperature equal to 1.1 Kelvin. This is basically the same critical temperature of aluminum (Al), which is the most used material for Josephson junctions implemented in qubits for quantum computer.

This prediction can open up new ways to realize ultra-thin superconducting films of noble metals, which will combine unique electronic and mechanical properties with experimentally accessible superconductivity.

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