Both cuprates and nickelates comprise 2D oxide planes (red, green and grey spheres represent copper, nickel and oxygen ions, respectively) separated by layers of a rare-earth material (gold spheres). Cuprates are inherent insulators, and even when they're doped to add free-flowing electrons (blue spheres), their electrons rarely leave to interact with other layers of the material. Nickelates, on the other hand, are inherent metals; even in the non-doped state, their electrons mix with electrons from the rare-earth layers in a way that creates a 3D metallic state. Image: Greg Stewart/SLAC National Accelerator Laboratory.
Both cuprates and nickelates comprise 2D oxide planes (red, green and grey spheres represent copper, nickel and oxygen ions, respectively) separated by layers of a rare-earth material (gold spheres). Cuprates are inherent insulators, and even when they're doped to add free-flowing electrons (blue spheres), their electrons rarely leave to interact with other layers of the material. Nickelates, on the other hand, are inherent metals; even in the non-doped state, their electrons mix with electrons from the rare-earth layers in a way that creates a 3D metallic state. Image: Greg Stewart/SLAC National Accelerator Laboratory.

The discovery last year of the first nickel oxide material showing clear signs of superconductivity set off a race by scientists around the world to find out more. The crystal structure of this nickel oxide material is similar to copper oxides, or cuprates, which hold the world record for conducting electricity with no loss at relatively high temperatures and normal pressures. But do its electrons behave in the same way?

The answers could help advance the synthesis of new unconventional superconductors and their use for power transmission, transportation and other applications, and also shed light on how the cuprates operate – which is still a mystery after more than 30 years of research.

Now, in a paper in Nature Materials, a team led by scientists at the US Department of Energy's SLAC National Accelerator Laboratory and Stanford University report the first detailed investigation into the electronic structure of superconducting nickel oxides, or nickelates.

The scientists used two techniques – resonant inelastic X-ray scattering (RIXS) and X-ray absorption spectroscopy (XAS) – to get the first complete picture of the nickelates' electronic structure – basically the arrangement and behavior of their electrons, which determine a material's properties.

Both cuprates and nickelates come in thin, two-dimensional sheets that are layered with other elements, such as rare-earth ions. These thin sheets become superconducting when they're cooled below a certain temperature and the density of their free-flowing electrons is adjusted via a process known as ‘doping’, which involves adding other elements to the sheets.

Cuprates are insulators in their pre-doped ‘ground’ states, meaning that their electrons are not mobile. After doping, their electrons can move about freely but are mostly confined to the cuprate layers, rarely traveling through the intervening rare-earth layers to reach their cuprate neighbors.

But in the nickelates, the team discovered, this is not the case. The undoped compound is a metal with freely flowing electrons. Furthermore, the intervening layers actually contribute electrons to the nickelate sheets, creating a three-dimensional metallic state that is quite different from what's seen in the cuprates.

This represents an entirely new type of ground state for transition metal oxides such as cuprates and nickelates, the researchers said. It opens new directions for experiments and theoretical studies of how superconductivity arises and how it can be optimized in this system and possibly in other compounds.

This story is adapted from material from the SLAC National Accelerator Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.