First superconductor made of nickel oxide

Exotic nickel compounds could indicate a whole new class of superconductors

By deliberately transforming a perovskite nickel oxide into a new, strontium-containing crystal structure, the researchers generated a new kind of supralreiter. © Danfeng Li / SLAC National Accelerator Laboratory, Stanford University
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Exotic crystal: Researchers have first demonstrated superconducting properties in a nickel oxide. At low temperatures, this strontium-containing nickelate lost its electrical resistance and conducted electrons lossless. Thus, this type of nickel compounds could form a whole new class of superconductors - and provide valuable insights into the mechanisms behind this exotic property, as the researchers report in the journal "Nature".

In superconductors, electrons can move with almost no energy loss, so these materials conduct electricity almost without resistance. Typically, this condition only occurs at temperatures close to absolute zero. However, there are some materials that become superconducting even at significantly higher temperatures, including graphene, metal hydrides, hydrogen sulphide and certain copper compounds, the so-called cuprates.

Nickel in the sights

But the researchers have not yet found the "holy grail" among the superconductors - a material that can conduct electrons lossless at room temperature and normal pressure. Among other things, they search in the periodic table for further elements whose compounds could show this property. The problem, however: why these materials are superconducting, is still only partially understood.

An especially promising candidate, Danfeng Li of the SLAC National Accelerator Laboratory in Menlo Park and his colleagues, has now studied: nickel, the element that is next to copper in the periodic table. This element can form oxides, so-called nickelates, which under certain conditions have a crystal structure very similar to the cuprates. That could turn them into good high temperature superconductors, so hope.

Shifting in the nickel oxide crystal

The problem, however, is that until now researchers have failed to bring the nickelates into the desired structure. "At the high temperatures that these materials normally produce, around 600 degrees, this nickelate is not stable, " explains Li. "So we had to start with a compound that we could generate stably at this heat and then this one at lower temperatures to the desired shape. "Display

For this, the researchers began with a neodymium-containing nickel oxide (NdNiO3), from which they grown ultra-thin crystal layers on a substrate made of strontium titanium oxide (SrTiO3). In several steps, they then converted this initial structure into a strontium-containing nickelate. The structure of this material changed so that an "infinite layer" phase was created a crystal structure that promotes superconductivity.

Superconducting, but different from Cuprate

And in fact, as the scientists continued to cool down and energize this nickelate, the electrical resistance suddenly collapsed. Below 14.5 Kelvin minus 258.6 degrees Celsius das, the strontium-doped nickelate became superconducting. This makes it clear that not only cuprates but also certain nickelates can become superconductors.

The exciting thing is that the nickelates differ in some electrochemical characteristics of the cuprate superconductors, so they are not magnetic, for example. This suggests that the physical basis for their loss of resistance may be different than the cup rates, as Li and his colleagues explain.

New class of superconductors?

"This is an important discovery because it forces us to rethink the details of the electronic structure and superconducting mechanisms of these materials, " writes George Sawatzky of the University of British Columbia in an accompanying commentary in "Nature." Both he and the research team think it is quite likely that behind this nickelate could hide a whole new kind and class of superconductors.

The transition temperature of nickelate, the transition to the superconductor, is still very low. But the scientists assume that, as with the cuprates, these nickel oxides could also form forms that form high-temperature superconductors. "We are still at the very beginning, " says senior author Harold Hwang of the SLAC. "There is still a lot of work and experiments ahead of us." (Nature, 2019; doi: 10.1038 / s41586-019-1496-5)

Source: DOE / SLAC National Accelerator Laboratory

- Nadja Podbregar