Superconducting materials are technologically important because electricity flows through them without resistance. Only low-temperature superconductivity seemed possible before 1986, but materials that superconduct at low temperatures are impractical because they must first be cooled to hundreds of degrees below zero. In 1986, discovery of high-temperature superconductivity in copper oxide compounds, called cuprates, engendered new technological potential for the phenomenon.
After three decades of ensuing research, exactly how cuprate superconductivity works remains a defining problem in the field. One approach to solving this problem has been to study compounds that have similar crystal, magnetic, and electronic structures to the cuprates. Nickel-based oxides — nickelates — have long been considered as potential cuprate analogs because the element sits immediately adjacent to copper in the periodic table.
A newly created nickel oxide compound was discovered as an unconventional, but promising candidate material for high-temperature superconductivity. The researchers synthesized single crystals of a metallic trilayer nickelate compound, which does not superconduct. It is poised, however, for superconductivity in a way not found in other nickel oxides if the right electron concentration can be determined.
The nickelate is a quasi-two-dimensional trilayer compound, meaning that it consists of three layers of nickel oxide separated by spacer layers of praseodymium oxide. This nickelate and a compound containing lanthanum, rather than praseodymium, both share the quasi-two-dimensional trilayer structure. But the lanthanum analog is non-metallic and adopts a so-called “charge-stripe” phase, an electronic property that makes the material an insulator — the opposite of a superconductor.
Crystals measuring a few millimeters in diameter were created, and the electronic structure of the nickelate was verified as resembling that of cuprate materials by taking X-ray absorption spectroscopy measurements, and by performing density functional theory calculations, which are used to investigate the electronic properties of condensed matter systems.
The next phase of the work will involve attempting to induce superconductivity in the nickelate material using a chemical process called electron doping, in which impurities are deliberately added to a material to influence its properties.