Non-Superconducting Metals Can Become Superconducting When Layered into Bilayers

A new understanding of nanoscale superconductivity emerges from theoretical work by Giovanni A. Ummarino and Alessio Zaccone at Politecnico di Torino, in collaboration with the National Research Nuclear University MEPhI, University of Milan, and University of G ottingen. The interplay between quantum confinement and proximity effects sharply increases the critical temperature for superconductivity within metallic heterostructures. Predictions indicate superconductivity appears in specific bilayer structures even if the constituent materials are not superconducting individually. These findings suggest quantum-confined bilayers represent a key pathway for designing new superconducting materials from metallic heterostructures.

Quantum confinement boosts superconductivity in metallic bilayer films

The superconducting critical temperature can be enhanced in certain bilayers, even when the constituent materials are nonsuperconducting or only weakly superconducting in bulk form. These findings identify quantum-confined bilayers as a promising route to engineering emergent superconductivity in metallic heterostructures. In crystalline superconducting films, such as lead and aluminium, quantum confinement has been shown to produce pronounced and often non-monotonic variations of the superconducting critical temperature with thickness, including enhancements above the bulk value.

Confinement effects also modify other electrodynamic properties, including the magnetic penetration depth and the superfluid density, further demonstrating that superconductivity in ultrathin films can differ substantially from the bulk behaviour. These phenomena originate from confinement-induced modifications of the electronic structure, particularly changes in the density of states and in the Fermi energy as the film thickness approaches the nanometer scale. Atomic-scale roughness and structural disorder at interfaces are a key aspect of quantum confinement in experimentally realised thin films.

Real materials exhibit irregular interfaces, unlike idealised models based on perfectly smooth boundaries, which prevent strict quantisation of the out-of-plane momentum kz. Consequently, electronic states do not become discretised into subbands, but instead undergo a continuous redistribution in momentum space. A recently developed quantum confinement theory explicitly incorporates this realistic scenario and provides analytical expressions for thickness-dependent electronic, phononic, and superconducting properties in excellent agreement with experiments. This approach therefore offers a physically grounded description of confinement effects in real heterostructures.

From an intuitive perspective, the confinement mechanism suppresses long-wavelength quasiparticle states along the direction perpendicular to the film. When film thickness is reduced, quasiparticles with wavelengths larger than the confinement length cannot propagate, effectively removing a portion of momentum space. This redistribution of electronic states near the Fermi surface modifies the density of states at the Fermi level. Even moderate confinement can lead to variations of the superconducting critical temperature, as superconductivity in conventional materials depends sensitively on this quantity.

This mechanism does not rely on idealised subband formation but instead arises naturally from realistic boundary conditions including surface roughness. Superconducting proximity effects in multilayer heterostructures also allow Cooper pairs to propagate across interfaces, enabling superconducting correlations to develop in adjacent metallic layers. When quantum confinement and proximity effects coexist, they may cooperate and lead to enhanced superconductivity or even to the emergence of superconductivity in systems composed of nonsuperconducting materials.

This work investigates the possibility of constructing two-layer heterostructures in which quantum confinement and proximity effects act simultaneously. Systems comprising bilayers where the individual components may be superconducting, weakly superconducting, or even normal metals in the bulk state are explored. Researchers combine confinement-induced modifications of the electronic structure with interlayer proximity coupling, to create new superconducting states with enhanced critical temperatures.

To address this problem, the generalised Eliashberg equations including both quantum confinement and proximity effects are employed. Three conditions must be satisfied simultaneously: (i) the system must consist of ultrathin layers of two different metals; (ii) the two materials must remain immiscible to avoid alloy formation; and (iii) the Fermi surface can be approximated as spherical, which applies to simple metals such as alkali metals, alkaline earth metals, noble metals, aluminium, and lead. Under these conditions, bilayer combinations that produce superconductivity with critical temperatures higher than those of the individual bulk materials are identified.

The model contains no adjustable parameters, since all input quantities are taken from experimentally known material properties of isotropic, one-band, low-Tc, s-wave superconductors. Other groups have also investigated this type of problem within different theoretical frameworks and for high-Tc superconductors. Within the extended Eliashberg framework, quantum confinement modifies the normal density of states (NDOS), the Fermi energy, and the effective interaction parameters entering the Eliashberg equations.

The NDOS, N(ε), becomes explicitly energy dependent, reflecting the confinement-induced reconstruction of the electronic spectrum. The Fermi energy is renormalised according to EF = C(L)2EF,bulk, where EF,bulk is the bulk Fermi energy and C(L) is a dimensionless confinement factor depending on the film thickness L. The effective electron, phonon coupling constant, λ, is modified as λ = C(L)λbulk, where λbulk is the electron, phonon coupling constant of the bulk material. The Coulomb pseudopotential μ∗becomes thickness dependent: μ∗(L) = C(L)μbulk 1 + μbulk ln[EF(L)/ωc], where μbulk is the bulk Coulomb pseudopotential and ωc is the cutoff frequency. The bandwidths WS(N) are taken as half of the corresponding Fermi energies, and quantum confinement modifies the material parameters in each layer: λS(N) = CS(N) λS(N),bulk, μ∗ S(N) = CS(N) μS(N),bulk.

Quantum confinement and proximity effects induce superconductivity in metallic bilayers

The theoretical work demonstrates the emergence of superconductivity in bilayer metallic heterostructures, effectively creating an infinite increase in superconducting potential for materials previously non-superconducting in bulk form. This breakthrough crosses a fundamental threshold, allowing superconductivity to be induced through nanoscale engineering rather than relying on inherently superconducting materials. Substantially enhanced critical temperatures have been predicted by combining quantum confinement, restricting the movement of electrons, and proximity effects, where superconductivity spreads between materials, potentially revolutionising energy transmission and storage.

Quantum confinement and proximity effects in bilayer metallic heterostructures can induce superconductivity in materials that are not superconducting in their bulk form. The theoretical framework employed incorporates modifications to the normal density of states, a measure of available electron states, which becomes energy dependent due to the restricted movement of electrons within the nanoscale layers. Furthermore, the Fermi energy, representing the highest energy electrons possess at absolute zero, is renormalised by a factor dependent on layer thickness; calculations show this factor is C(L)², indicating a significant alteration of electronic properties.

Engineering superconductivity in metallic bilayers via quantum confinement and proximity effects

Finding new superconducting materials is crucial for lossless energy transfer and revolutionary computing. This theoretical work offers a compelling route, suggesting superconductivity can be engineered into bilayers, two layers of different metals joined together, by manipulating quantum effects. However, the Eliashberg framework used relies on a simplified assumption of spherical Fermi surfaces, limiting its direct applicability to more complex materials with intricate electronic structures.

Acknowledging limitations in the Eliashberg framework’s simplified assumptions is vital for refining future models. This theoretical work nonetheless establishes a promising pathway for designing novel superconducting materials by combining quantum confinement with proximity effects. Specifically, bilayers offer a flexible platform to engineer superconductivity even when individual components lack this property. This theoretical investigation confirms that bilayer metallic heterostructures offer a pathway to induce superconductivity even in materials that do not exhibit this property in bulk form, with the critical temperature substantially enhanced by simultaneously exploiting quantum confinement and proximity effects, shifting the focus from discovery to precise structural engineering.

The research demonstrated that superconductivity could be induced in bilayer metallic heterostructures, even when the individual metals are not superconducting on their own. This matters because it presents a new strategy for creating superconducting materials through structural design rather than relying on the discovery of new elements. By combining quantum confinement, restricting electron movement in nanoscale layers, with proximity effects, the critical temperature for superconductivity was theoretically enhanced. Future work could focus on applying this framework to more complex materials and validating these predictions through experimental fabrication and characterisation of these bilayer structures.