Physicists in the US have created the world’s thinnest high-temperature superconductor, demonstrating that the phenomenon can exist over a thickness of a few atoms.
Gennady Logvenov and colleagues at Brookhaven National Laboratory in Upton, New York, have created layered films of copper-oxide or “cuprate” materials and have discovered that they can localize the superconducting behaviour to a single atomic plane. They say that the discovery will help theorists to build more comprehensive models of high-temperature superconductivity, and lead to thin-film devices that have their superconducting properties tuned by electric fields.
“We wanted to answer a fundamental question about such films,” says team member Ivan Bozovik. “Namely: how thin can the film be and still retain high-temperature superconductivity?”
No resistance
Discovered at the beginning of the 20th century, superconductivity is a phenomenon whereby a material’s electrical resistance can suddenly drop to zero as the substance is chilled below a specific temperature – known as the transition temperature (Tc). It exists in some pure metals close to absolute zero, and scientists believe that this is because electrons distort the metal lattice to let subsequent electrons flow freely, a mechanism outlined in so-called Bardeen-Cooper-Schrieffer (BCS) theory.
In 1986, however, physicists discovered that superconductivity also exists in certain compounds, including cuprates, at much higher temperatures of 30 K and more. This discovery of high-Tc superconductivity triggered a lot of initial excitement due to suggestions that, if extended up to room temperature, it could lead to novel applications such as levitating trains and ultra-efficient power cables. Over the past 20 years, however, these exciting new technologies have not materialized because physicists and engineers have struggled to understand the mechanism behind the phenomenon.
Now, Logvenov and colleagues have performed an experiment that could help to point theorists in the right direction. They have created a “bilayer” film with one layer of a cuprate metal and another of a cuprate insulator, using a technique called molecular beam epitaxy. Superconductivity in such bilayers tends to manifest at the interface between the layers, so the researchers were able to isolate where the effect occurs by carefully doping atomic planes within the layers with zinc, which suppresses superconductivity.