A new way of making high-temperature superconductors that is based on metamaterials has been proposed by physicists in the US. Their plan involves combining a low-temperature superconductor with a dielectric material to create a metamaterial that is a superconductor at much higher temperatures than its constituent materials. The team is now looking at testing its proposal in the lab and is hopeful that its work could offer a route to creating a superconductor that operates at room temperature.

Ever since the first high-temperature superconductor was discovered nearly 30 years ago, physicists have searched in vain for a material that remains a superconductor at room temperature. But despite a massive effort, physicists have not been able to create a superconductor that endures at temperatures higher than about 140 K, which is still 150 degrees below room temperature.

Now Vera Smolyaninova of Towson University and Igor Smolyaninov of the University of Maryland have proposed a new approach to creating a superconductor with a high critical temperature (Tc) – the temperature above which the material ceases to be superconducting. Their proposal involves creating man-made structures called metamaterials, which can be engineered to have electromagnetic properties that are not normally found in nature. This includes negative indices of refraction, which have been used to create devices such as invisibility cloaks and super lenses.

The pair's proposal is inspired by a description of superconductivity that was derived in 1973 by the Russian physicist David Kirzhnits and colleagues. Kirzhnits' approach is complementary to the conventional theory of superconductivity and it points out that the strength of the interaction between electrons in a superconductor is inversely proportional to the dielectric response (ε) of the material.

Maximum attraction

Conventional superconductivity arises when there is an attraction between electrons, causing them to form pairs. If a metamaterial can be engineered with a small and negative value of ε, it would have a larger attractive interaction between electrons and therefore stand a good chance of being a superconductor with a relatively high Tc.

In a preprint on the arXiv server, Smolyaninova and Smolyaninov argue that the ε-near-zero (ENZ) approach to designing metamaterials could offer a blueprint for creating a material with the appropriate value of ε. ENZ metamaterials are mixtures of metallic and dielectric components and in their proposal the metal is also a conventional superconductor – these are metals such as lead and mercury that have Tc values below 10 K.

The ENZ metamaterial proposed by the researchers involves making a superconductor with random "inclusions" of dielectric material. Smolyaninova told physicsworld.com that a possible candidate for the dielectric is the ferroelectric material strontium titanate, which can be made in nanoparticle form. The sizes of the inclusions and typical distances between them must be smaller than the correlation length between electron pairs in the superconductor – which is about 100 nm.

Hyperbolic design

Another design proposed by the team is a "hyperbolic" metamaterial in which the desired ε is engineered using alternating layers of metallic and dielectric materials. Indeed, the researchers point out that typical high-Tc superconductors do share some properties with hyperbolic metamaterials. Again, the metal would be a conventional superconductor.

"We are working on actual metamaterial designs and preparing actual experiments now," Smolyaninova said. She adds that the ENZ design would be easier to implement than the hyperbolic metamaterial.

Smolyaninova is hopeful that metamaterial superconductors could be made with Tc values above the boiling temperature of liquid nitrogen (77 K). This would make them appropriate for use in systems that currently use high-Tc superconductors.