Superconductors are among the wonders of modern science. They allow a current to flow with zero resistance in materials cooled below some critical temperature. Superconductors are the crucial ingredients in everything from high-power magnets and MRI machines to highly sensitive magnetometers and magnetic levitation devices.

One problem though is that superconductors work only at very low temperatures. So one of the great challenges in this area of science is to find materials that superconduct at higher temperatures, perhaps even at room temperature. That won’t be easy given that the current record is around 150 kelvin (-120 degrees centigrade).

Nevertheless, a way of increasing the critical temperature of existing superconducting materials would be hugely useful.

Today, a group of physicists and engineers say they have worked out how to do this. The trick is to think of a superconductor as a special kind of metamaterial and then to manipulate its structure in a way that increases its critical temperature.

Vera Smolyaninova at Towson University in Maryland and colleagues from the University of Maryland and the Naval Research Laboratory in Washington DC, have even demonstrated this idea by increasing the critical superconducting temperature of tin.

First some background about metamaterials. Until relatively recently, physicists had always treated bulk materials as homogeneous lumps of the same stuff. These lumps have bulk properties such as the ability to bend light in a certain way.

But in recent years they have began to think about constructing artificial materials made of periodic patterns of structures that themselves interact with electromagnetic waves, things like wires, c-shaped conductors and so on. If these structures are much smaller than the wavelength of the light passing by, then they act like a homogeneous lump, at least as far as the light is concerned.

By toying with this periodic structure, physicists can create artificial materials with all kinds of exotic properties. The most famous of these is the invisibility cloak, a metamaterial designed to steer light around an object as if it were not there.

Superconductivity can be thought of in a similar way, say Smolyaninova and co. Conventional superconductors made of a single metal are homogeneous lumps of the same stuff that have zero resistance at some critical temperature.

But in the 1980s, physicists discovered an entirely new class of superconductor made of much more complex arrangements of atoms but which superconduct at much higher temperatures. According to Smolyaninova and co, some of these are remarkably similar to the metamaterials that physicists have built to steer light.

For example, materials known as BSCCOs (bismuth-strontium-calcium-copper oxides) superconduct at over 100 Kelvin but they are not homogeneous lumps at all. Instead they are made up of layers of metals and dielectrics. It is these layers that help to guide electrons through the structure without resistance, rather like the active structures in metamaterials designed to steer light.

That gave Smolyaninova and co an idea. Maybe a conventional superconductor such as tin can be turned into superconducting metamaterial by the addition of a dielectric such as barium titanate. They even calculated that this kind of structure should superconduct at a higher temperature than the tin alone, provided that the dielectric was smaller than some critical scale.

That’s certainly an interesting theory but little more than a curiosity in the absence of any experimental verification. Now, that is exactly what Smolyaninova and co can provide.

These guys mixed nanoparticles of tin with nanoparticles of barium titanate in a single test tube filled with deionised water. After sufficient stirring, they allowed the water to evaporate and compressed the remaining mixture into pellets. By changing the proportion of tin and barium titanate nanoparticles, they hoped to change the superconducting properties of the pellets.

Sure enough, that is exactly what they have measured. As the proportion of barium titanate increases, the superconducting threshold rises. The maximum effect occurs with the pellets contain 40 per cent barium titanate when the critical temperature is 0.15 Kelvin higher than in tin alone.

“An increase of the critical temperature of the order of 0.15 K compared to bulk tin has been observed for [approximately] 40% volume fraction of barium titanate nanoparticles,” say Smolyaninova and co.

That is a potentially important result. Solid-state physicists have been searching for ways to increase the superconducting temperature of the materials they play with for many years. However, much of this work is done using little more than inspired guesses.

For the first time, Smolyaninova and co provide a rationale behind this process that can be used to engineer superconducting metamaterials with higher critical temperatures.

Of course, it is early days for these guys. Tin superconducts at a temperature of only 3.7 Kelvin and an increase of 0.15 Kelvin is tiny. And Smolyaninova and co need to determine the structure of their pellets to confirm that the material works as a metamaterial in the way they think. This is clearly a stepping stone to more advanced experiments that attempt to engineer more complex structures into superconducting materials.

It also represents something of a challenge to the theory of high-temperature superconductivity, which has puzzled physicists since it was first discovered. Nobody is quite sure why these materials work at such high temperatures. But they are certain that whatever the reason, the physics is entirely different from that which describes conventional superconductivity with materials like tin.

Just what the link might be between conventional superconductivity, high-temperature superconductivity and metamaterial physics is a fascinating open question that might bear fruit given the right kind of nurturing.

One thing is for sure — you can expect to see other researchers attempting to verify this work and take metamaterial superconductors to new heights of high temperature performance.

Ref: arxiv.org/abs/1408.0704 : Experimental Demonstration Of Superconducting Critical Temperature Increase In Electromagnetic Metamaterials