Superconductivity found in a nickel oxide compound

In 1986 Georg Bednorz and Alex Müller discovered superconductivity in an oxide of lanthanum, barium, and copper—La_1.85Ba_0.15CuO₄. The achievement won the researchers a Nobel Prize the following year and triggered an explosion of research in condensed-matter physics (see Physics Today, December 1987, page 17 ). Yet despite 33 years of subsequent research, no consensus has emerged as to how the cuprates work.

To gain insight, researchers have proposed building solids that incorporate similar structural, magnetic, and electronic features—a two-dimensional lattice and d- and p-orbital hybridization among them—in hopes that superconductivity would emerge. Replacing Cu with another transition metal was one obvious path. Nickel sits next to Cu in the periodic table, and in 1999 theorists predicted that if Ni⁺¹ could be synthesized in the LaNiO₂ (nickelate) lattice, it would have the same electronic configuration as the cuprates: Both Ni⁺¹ and Cu⁺² would have a single hole in the outer d shell. Harold Hwang , his postdoc Danfeng Li, and their colleagues at SLAC and Stanford University have now successfully synthesized a doped version of such a nickelate: neodymium strontium nickel oxide. What’s more, they found that it superconducts.

The Stanford group’s central insight was to chemically dope and grow a precursor compound, NdNiO₃, with holes (via strontium substitution) before reducing it to create Nd_0.8Sr_0.2NiO_2. Prior groups had doped the nickelate or reduced it, but not both. The new method accomplished two purposes: It produced the sought-after oxidation state, and it created an “infinite layer” phase of NiO₂ planes—an analogue of the cuprates’ CuO₂ planes. Although the material was found to superconduct only below 9–15 K, a small transition temperature by cuprate standards, half a dozen theory papers have appeared on arXiv.org just three weeks after the researchers’ publication. No doubt they’re just the early efforts to understand the similarities and differences between the superconductors. (D. Li et al., Nature 572, 624, 2019 .)