Materials scientists at the U.S. Department of Energy’s Argonne National Laboratory have identified the trilayer nickelate compound Pr4Ni3O8 as a promising candidate material for high-temperature superconductivity. The findings are published in the journal Nature Physics.
A crystal of the trilayer nickelate compound Pr4Ni3O8. Image credit: Argonne National Laboratory.
“It’s poised for superconductivity in a way not found in other nickel oxides. We’re very hopeful that all we have to do now is find the right electron concentration,” said senior author Dr. John Mitchell, associate director of the Argonne National Laboratory’s Materials Science Division.
“This nickel oxide compound does not superconduct, but it’s poised for superconductivity in a way not found in other nickel oxides. We’re very hopeful that all we have to do now is find the right electron concentration.”
Superconducting materials are technologically important because electricity flows through them without resistance.
High-temperature superconductors could lead to faster, more efficient electronic devices, grids that can transmit power without energy loss and ultra-fast levitating trains that ride frictionless magnets instead of rails.
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, however, discovery of high-temperature superconductivity in copper oxide compounds called cuprates engendered new technological potential for the phenomenon.
But 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.
“Thus far, that’s been an unsuccessful quest. None of these analogs have been superconducting, and few are even metallic,” Dr. Mitchell said.
The nickelate that Dr. Mitchell and co-authors created, Pr4Ni3O8, consists of three layers of nickel oxide that are separated by spacer layers of praseodymium oxide.
“Thus it looks more two-dimensional than three-dimensional, structurally and electronically,” Dr. Mitchell said.
Pr4Ni3O8 and a compound containing lanthanum rather than praseodymium, La4Ni3O8, 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.
“For some yet-unknown reason, the praseodymium system does not form these stripes. It remains metallic and so is certainly the more likely candidate for superconductivity,” Dr. Mitchell said.
“We didn’t know for sure we could make these materials,” said first author Junjie Zhang, postdoctoral researcher at Argonne National Laboratory.
But indeed, the researchers managed to grow the crystals measuring a few millimeters in diameter.
They verified that the electronic structure of Pr4Ni3O8 resembles that of cuprate materials by taking X-ray absorption spectroscopy measurements and by performing density functional theory calculations.
_____
Junjie Zhang et al. Large orbital polarization in a metallic square-planar nickelate. Nature Physics, published online June 12, 2017; doi: 10.1038/nphys4149
