1.85 Ba 0.15 CuO 4 . The achievement won the researchers a Nobel Prize the following year (see Physics Today, T c of subsequent cuprates exceed those of any previously known superconductor by almost an order of magnitude. Yet despite 33 years of research since then, no consensus has emerged as to what causes their superconductivity. In 1986 Georg Bednorz and Alex Müller discovered superconductivity in an oxide of lanthanum, barium, and copper—LaBaCuO. The achievement won the researchers a Nobel Prize the following year (see December 1987, page 17 ) and triggered an explosion of research in condensed-matter physics. Although that oxide superconducts below a relatively low 30 K, the transition temperaturesof subsequent cuprates exceed those of any previously known superconductor by almost an order of magnitude. Yet despite 33 years of research since then, no consensus has emerged as to what causes their superconductivity.

0.8 Sr 0.2 NiO 2 , which superconducts below 15 K. 1 et al. , Nature 572, 624 (2019). 1. D. Li, 624 (2019). https://doi.org/10.1038/s41586-019-1496-5 Prospects appear brighter now that a new family of cuprate-like superconductors has been realized. Harold Hwang, his postdoc Danfeng Li, and their colleagues at SLAC and Stanford University have successfully synthesized neodymium strontium nickel oxide, NdSrNiO, which superconducts below 15 K.

Hwang’s group did not stumble on the superconducting nickelates purely by serendipity. Rather, their quest was inspired by a theoretical prediction that was in turn informed by what experimenters and theorists have learned about cuprates over the years. Despite their different chemistry, near the Fermi level Ni and Cu have an apparently similar electronic structure, which is dominated by a single d x 2 − y 2 orbital. The differences of detail between the families should shed light on the origins of superconductivity.

Mott insulators Section: Choose Top of page ABSTRACT Mott insulators << Long time coming In search of a mechanism References 2 planes are separated by charge reservoirs. Each unit cell in the CuO 2 plane has an odd number of electrons, and their states are so well localized that it takes a large amount of energy for an electron to hop from one lattice site to another. Indeed, the cuprates are materials whose single-particle band structure tells you should be metals but are, in fact, Mott insulators because of electron–electron repulsion that creates a traffic jam. 2 et al. , Nature 518, 179 (2015). 2. B. Keimer, 179 (2015). https://doi.org/10.1038/nature14165 At room temperature the cuprates are such poor conductors that they barely qualify as metals. Their stacks of closely spaced CuOplanes are separated by charge reservoirs. Each unit cell in the CuOplane has an odd number of electrons, and their states are so well localized that it takes a large amount of energy for an electron to hop from one lattice site to another. Indeed, the cuprates are materials whose single-particle band structure tells you should be metals but are, in fact, Mott insulators because of electron–electron repulsion that creates a traffic jam. The magnetic moments of the material’s nearly filled Cu2+ 3d9 shell arrange themselves in a two-dimensional checkerboard with strong antiferromagnetic interactions between neighboring spin-½ Cu ions, each separated by an O ion. The usual approach to studying the cuprates’ peculiar superconductivity is to modify the charge-carrier concentration in the CuO 2 planes through chemical doping. (For instance, one could introduce holes by substituting Ba2+ for La3+.) Hole doping suppresses the antiferromagnetic order, and superconductivity sets in at a critical doping concentration. d- and p-orbital hybridization among them. Replacing Cu with another transition metal was an obvious path. 3 Rep. Prog. Phys. 79, 074502 (2016); et al. , Nat. Phys. 13, 864 (2017). 3. M. Norman,, 074502 (2016); https://doi.org/10.1088/0034-4885/79/7/074502 see also J. Zhang, 864 (2017). https://doi.org/10.1038/nphys4149 2+ in the cuprates. 4 Phys. Rev. B 59, 7901 (1999). 4. V. I. Anisimov, D. Bukhvalov, T. M. Rice,, 7901 (1999). https://doi.org/10.1103/PhysRevB.59.7901 d shell. Soon after the cuprates were discovered, Princeton University’s Philip Anderson argued that their superconductivity is somehow inherited from the properties of a doped Mott insulator. One strategy for gaining further insight was to look for superconductivity in solids that incorporate similar structural, magnetic, and electronic features—a 2D lattice, spin-½ ions, and- and-orbital hybridization among them. Replacing Cu with another transition metal was an obvious path.Nickel sits next to Cu in the periodic table, and theorists Vladimir Anisimov, Danil Bukhvalov, and Maurice Rice predicted in 1999 that if Ni could be synthesized in the unusual +1 oxidation state in a lanthanum nickelate lattice, it would have the same electronic configuration as Cuin the cuprates.Each would have a single hole in its 3shell. 2 , Hwang’s group finally found a superconducting analogue. Although the transition temperature of 15 K is meager by cuprate standards, the achievement has generated enormous enthusiasm. Just four weeks after the researchers’ publication, 1 et al. , Nature 572, 624 (2019). 1. D. Li, 624 (2019). https://doi.org/10.1038/s41586-019-1496-5 By partially substituting strontium for neodymium in NdNiO, Hwang’s group finally found a superconducting analogue. Although the transition temperature of 15 K is meager by cuprate standards, the achievement has generated enormous enthusiasm. Just four weeks after the researchers’ publication,more than a dozen theory papers had appeared on arXiv.org

Long time coming Section: Choose Top of page ABSTRACT Mott insulators Long time coming << In search of a mechanism References 2 compounds as powders and thin films. The first synthesis was done in the early 1980s, before Bednorz and Müller’s award-winning cuprate work. Nickelates ordinarily prefer an octahedral coordination—a network of Ni atoms surrounded by four oxygens in one plane and two “apical” oxygens above and below it. In 1983 chemists Michel Crespin, Pierre Levitz, and Lucien Gatineau realized they could start with that phase—a 3D perovskite LaNiO 3 —and expose it to hydrogen gas to reduce it into LaNiO 2 with a 2D planar geometry. 5 J. Chem. Soc., Faraday Trans. 2 79, 1181 (1983). 5. M. Crespin, P. Levitz, L. Gatineau,, 1181 (1983). https://doi.org/10.1039/F29837901181 Several groups have made LaNiOcompounds as powders and thin films. The first synthesis was done in the early 1980s, before Bednorz and Müller’s award-winning cuprate work. Nickelates ordinarily prefer an octahedral coordination—a network of Ni atoms surrounded by four oxygens in one plane and two “apical” oxygens above and below it. In 1983 chemists Michel Crespin, Pierre Levitz, and Lucien Gatineau realized they could start with that phase—a 3D perovskite LaNiO—and expose it to hydrogen gas to reduce it into LaNiOwith a 2D planar geometry. In the perovskite phase, planes of LaO alternate with those of NiO 2 . Reducing the perovskite strips out about a third of the oxygens (the apicals) while leaving the NiO 2 framework, whose planes are then separated only by La atoms. The layered structure (LaNiO 2 ) that remains has both the square-planar geometry and a transition-metal oxidation state present in the cuprates. 2 gas with a metal-hydride reducing agent, which turned out to be safer and more reliable. 6 et al. , J. Amer. Chem. Soc. 121, 8843 (1999). 6. M. A. Hayward, 8843 (1999). https://doi.org/10.1021/ja991573i 7 et al. , Appl. Phys. Lett. 94, 082102 (2009). 7. M. Kawai, 082102 (2009). https://doi.org/10.1063/1.3078276 3 ) substrate, the reactions became more tractable. The 1983 synthesis and most others that followed produced polycrystalline powders. The large surface-to-volume ratios and random orientations of the crystals complicated the reduction chemistry: Reactions sometimes introduced Ni-metal inclusions and other defects or led to decomposition. A major step forward was to replace Hgas with a metal-hydride reducing agent, which turned out to be safer and more reliable.But it wasn’t until 2009 that Kyoto University’s Masanori Kawai and coworkers epitaxially grew the reduced planar structure as a single-crystal thin film.With the film grown on a strontium titanite (SrTiO) substrate, the reactions became more tractable. Hwang, Li, and their colleagues used the Kyoto group’s recipe as a springboard. They improved it in key ways: First, they swapped out La for Nd to make the material more conductive. Nd ions are smaller than La ions, and they shrink the nickelate’s in-plane lattice constant. The Stanford group also chemically doped the starting perovskite material with holes by substituting 20% of the Nd3+ ions with Sr2+. Earlier groups had doped the nickelate or reduced it, but not both. (An unpublished account of a doped, reduced sample was reported in Oxford University chemist Mike Hayward’s 1999 thesis, but no superconductivity was reported.) 3 lattice at the high temperature—600 °C—needed for it to crystallize atop SrTiO 3 did they reduce it. That step took place at a much lower temperature, 280 °C, and produced the layered phase shown in figure 1 2. The sequence also mattered. Only after the group had grown the Sr-doped NdNiOlattice at the high temperature—600 °C—needed for it to crystallize atop SrTiOdid they reduce it. That step took place at a much lower temperature, 280 °C, and produced the layered phase shown in figure. The resulting samples measure 2.5 × 5 mm