Superconductivity is a phenomenon of zero electrical resistance, and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature. The usual temperature is very close to absolute zero and far from room conditions. So the key is finding a ways to increase that temperature as high as one can. A multi-university team of researchers has artificially engineered a unique multilayer material that could lead to breakthroughs in both superconductivity research and in real-world applications. The researchers can tailor the material, which seamlessly alternates between metal and oxide layers, to achieve extraordinary superconducting properties—in particular, the ability to transport much more electrical current than non-engineered materials.
High-temperature superconductors are materials that behave as superconductors at unusually high temperatures. Until 1986, physicists had believed that BCS theory forbade superconductivity at temperatures above about 30 K. Since about 1993, the highest temperature superconductor was a ceramic material consisting of thallium, mercury, copper, barium, calcium and oxygen (HgBa2Ca2Cu3O8+δ) with Tc = 133—138 K. Please note this is only -210 degrees F!
An unconventional high-temperature superconductor, the researchers' iron-based pnictide material is promising in part because its effective operating temperature is higher than that of conventional superconducting materials such as niobium, lead or mercury. The research team engineered and measured the properties of superlattices of pnictide superconductors. A superlattice is the complex, regularly repeating geometric arrangement of atoms—its crystal structure—in layers of two or more materials.
Pnictide superconductors include compounds made from any of five elements in the nitrogen family of the periodic table. The researchers' new material is composed of 24 layers that alternate between the pnictide superconductor and a layer of the oxide strontium titanate. Creating such systems is difficult, especially when the arrangement of atoms, and chemical compatibility, of each material is very different.
Yet, layer after layer, the researchers maintained an atomically sharp interface—the region where materials meet. Each atom in each layer is precisely placed, spaced and arranged in a regularly repeating crystal structure. The new material also has improved current-carrying capabilities. As they grew the superlattice, the researchers also added a tiny bit of oxygen to intentionally insert defects every few nanometers in the material. These defects act as pinning centers to immobilize tiny magnetic vortices that, as they grow in strength in large magnetic fields, can limit current flow through the superconductor.
"If the vortices move around freely, the energy dissipates, and the superconductor is no longer lossless," says Eom. "We have engineered both vertical and planar pinning centers, because vortices created by magnetic fields can be in many different orientations." Eom sees possibilities for researchers to expand upon his team's success in engineering man-made superconducting structures.
"There's a need to engineer superlattices for understanding fundamental superconductivity, for potential use in high-field and electronic devices, and to achieve extraordinary properties in the system," says Eom. "And, there is indication that interfaces can be a new area of discovery in high-temperature superconductors. This material offers those possibilities."
For further information see Layer Superconductors.
Superconductor image via Wikipedia.