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Since the discovery of high-temperature superconductivity in copper oxide materials 1 , there have been sustained efforts to both understand the origins of this phase and discover new cuprate-like superconducting materials 2 . One prime materials platform has been the rare-earth nickelates and, indeed, superconductivity was recently discovered in the doped compound Nd 0.8 Sr 0.2 NiO 2 (ref. 3 ). Undoped NdNiO 2 belongs to a series of layered square-planar nickelates with chemical formula Nd n +1 Ni n O 2 n +2 and is known as the ‘infinite-layer’ ( n  =  ∞ ) nickelate. Here we report the synthesis of the quintuple-layer ( n  = 5) member of this series, Nd 6 Ni 5 O 12 , in which optimal cuprate-like electron filling ( d 8.8 ) is achieved without chemical doping. We observe a superconducting transition beginning at ~13 K. Electronic structure calculations, in tandem with magnetoresistive and spectroscopic measurements, suggest that Nd 6 Ni 5 O 12 interpolates between cuprate-like and infinite-layer nickelate-like behaviour. In engineering a distinct superconducting nickelate, we identify the square-planar nickelates as a new family of superconductors that can be tuned via both doping and dimensionality. The authors report a superconducting transition beginning at 13 K in films of the quintuple-layer nickelate Nd 6 Ni 5 O 12 .

The layered square-planar nickelates, Nd n +1 Ni n O 2 n +2 , are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd 6 Ni 5 O 12 thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n  = 3 Ruddlesden-Popper compound, Nd 4 Ni 3 O 10 , and subsequent reduction to the square-planar phase, Nd 4 Ni 3 O 8 . We synthesize our highest quality Nd 4 Ni 3 O 10 films under compressive strain on LaAlO 3 (001), while Nd 4 Ni 3 O 10 on NdGaO 3 (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd 4 Ni 3 O 10 on SrTiO 3 (001). Films reduced on LaAlO 3 become insulating and form compressive strain-induced c -axis canting defects, while Nd 4 Ni 3 O 8 films on NdGaO 3 are metallic. This work provides a pathway to the synthesis of Nd n +1 Ni n O 2 n +2 thin films and sets limits on the ability to strain engineer these compounds via epitaxy. The discovery of superconductivity in the infinite-layer nickelates reignites an interest in the nickelates as cuprate analogues. Here, the authors investigate the role of epitaxial strain in the synthesis of the n=3 layered nickelate, Nd 4 Ni 3 O 8 .

A key open question in the study of layered superconducting nickelate films is the role that hydrogen incorporation into the lattice plays in the appearance of the superconducting state. Due to the challenges of stabilizing highly crystalline square planar nickelate films, films are prepared by the deposition of a more stable parent compound which is then transformed into the target phase via a topotactic reaction with a strongly reducing agent such as CaH 2 . Recent studies, both experimental and theoretical, have introduced the possibility that the incorporation of hydrogen from the reducing agent into the nickelate lattice may be critical for the superconductivity. In this work, we use secondary ion mass spectrometry to examine superconducting La 1− x X x NiO 2 / SrTiO 3 ( X = Ca and Sr) and Nd 6 Ni 5 O 12 / NdGaO 3 films, along with non-superconducting NdNiO 2 / SrTiO 3 and (Nd,Sr)NiO 2 / SrTiO 3 . We find no evidence for extensive hydrogen incorporation across a broad range of samples, including both superconducting and non-superconducting films. Theoretical calculations indicate that hydrogen incorporation is broadly energetically unfavorable in these systems, supporting our conclusion that extensive hydrogen incorporation is not generally required to achieve a superconducting state in layered square-planar nickelates. The role of hydrogen in engendering superconductivity in layered nickelates is under intense debate. Here, the authors perform secondary ion mass spectroscopy and see no evidence for extensive hydrogen incorporation into superconducting nickelates.

Long viewed as passive elements, antiferromagnetic materials have emerged as promising candidates for spintronic devices due to their insensitivity to external fields and potential for high-speed switching. Recent work exploiting spin and orbital effects has identified ways to electrically control and probe the spins in metallic antiferromagnets, especially in non-collinear or non-centrosymmetric spin structures. The rare-earth nickelate NdNiO3 is known to be a non-collinear antiferromagnet in which the onset of antiferromagnetic ordering is concomitant with a transition to an insulating state. Here we find that for low electron doping, the magnetic order on the nickel site is preserved, whereas electronically, a new metallic phase is induced. We show that this metallic phase has a Fermi surface that is mostly gapped by an electronic reconstruction driven by bond disproportionation. Furthermore, we demonstrate the ability to write to and read from the spin structure via a large zero-field planar Hall effect. Our results expand the already rich phase diagram of rare-earth nickelates and may enable spintronics applications in this family of correlated oxides.Films of the correlated oxide NdNiO3 form a metallic antiferromagnetic phase that can be identified using electrical currents, raising the prospect of applications in spintronics.

In the Acknowledgements section of this article the grant number relating to NSF was incorrectly given as DMR 2045826 and should have been DMR-2045826. The original article has been corrected.

Magnetic interactions are thought to play a key role in the properties of many unconventional superconductors, including cuprates, iron pnictides, and square-planar nickelates. Superconductivity was also recently observed in the bilayer and trilayer Ruddlesden-Popper nickelates, whose electronic structure is expected to differ from that of cuprates and square-planar nickelates. Here we study how electronic structure and magnetic interactions evolve with the number of layers, \\(n\\), in thin film Ruddlesden-Popper nickelates Nd\\(_{n+1}\\)Ni\\(_{n}\\)O\\(_{3n+1}\\) with \\(n=1,\\:3\\), and 5 using resonant inelastic x-ray scattering (RIXS). The RIXS spectra are consistent with a high-spin \\(|3d^8 \\underline{L} \\rangle\\) electronic configuration, resembling that of La\\(_{2-x}\\)Sr\\(_x\\)NiO\\(_4\\) and the parent perovskite, NdNiO\\(_3\\). The magnetic excitations soften to lower energy in the structurally self-doped, higher-\\(n\\) films. Our observations confirm that structural tuning is an effective route for altering electronic properties, such as magnetic superexchange, in this prominent family of materials.

We examine the bulk electronic structure of Nd3Ni2O7 using Ni 2p core-level hard x-ray photoemission spectroscopy combined with density functional theory + dynamical mean-field theory. Our results reveal a large deviation of the Ni 3d occupation from the formal Ni2.5+ valency, highlighting the importance of the charge-transfer from oxygen ligands. We find that the dominant d8 configuration is accompanied by nearly equal contributions from d7 and d9 states, exhibiting an unusual valence state among Ni-based oxides. Finally, we discuss the Ni dx2-y2 and dz2 orbital-dependent hybridization, correlation and local spin dynamics.

The layered square-planar nickelates, Nd\\(_{n+1}\\)Ni\\(_{n}\\)O\\(_{2n+2}\\), are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd\\(_{6}\\)Ni\\(_{5}\\)O\\(_{12}\\) thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the \\(n=3\\) Ruddlesden-Popper compound, Nd\\(_{4}\\)Ni\\(_{3}\\)O\\(_{10}\\), and subsequent reduction to the square-planar phase, Nd\\(_{4}\\)Ni\\(_{3}\\)O\\(_{8}\\). We synthesize our highest quality Nd\\(_{4}\\)Ni\\(_{3}\\)O\\(_{10}\\) films under compressive strain on LaAlO\\(_{3}\\) (001), while Nd\\(_{4}\\)Ni\\(_{3}\\)O\\(_{10}\\) on NdGaO\\(_{3}\\) (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd\\(_{4}\\)Ni\\(_{3}\\)O\\(_{10}\\) on SrTiO\\(_{3}\\) (001). Films reduced on LaAlO\\(_{3}\\) become insulating and form compressive strain-induced \\(c\\)-axis canting defects, while Nd\\(_{4}\\)Ni\\(_{3}\\)O\\(_{8}\\) films on NdGaO\\(_{3}\\) are metallic. This work provides a pathway to the synthesis of Nd\\(_{n+1}\\)Ni\\(_{n}\\)O\\(_{2n+2}\\) thin films and sets limits on the ability to strain engineer these compounds via epitaxy.

The rare-earth nickelates possess a diverse set of collective phenomena including metal-to-insulator transitions, magnetic phase transitions, and, upon chemical reduction, superconductivity. Here, we demonstrate epitaxial stabilization of layered nickelates in the Ruddlesden-Popper form, Nd\\(_{n+1}\\)Ni\\(_n\\)O\\(_{3n+1}\\), using molecular beam epitaxy. By optimizing the stoichiometry of the parent perovskite NdNiO\\(_3\\), we can reproducibly synthesize the \\(n = 1 - 5\\) member compounds. X-ray absorption spectroscopy at the O \\(K\\) and Ni \\(L\\) edges indicate systematic changes in both the nickel-oxygen hybridization level and nominal nickel filling from 3\\(d^8\\) to 3\\(d^7\\) as we move across the series from \\(n = 1\\) to \\(n = \\infty\\). The \\(n = 3 - 5\\) compounds exhibit weakly hysteretic metal-to-insulator transitions with transition temperatures that depress with increasing order toward NdNiO\\(_3\\) (\\(n = \\infty)\\).

Long viewed as passive elements, antiferromagnetic materials have emerged as promising candidates for spintronic devices due to their insensitivity to external fields and potential for high-speed switching. Recent work exploiting spin and orbital effects has identified ways to electrically control and probe the spins in metallic antiferromagnets, especially in noncollinear or noncentrosymmetric spin structures. The rare earth nickelate NdNiO3 is known to be a noncollinear antiferromagnet where the onset of antiferromagnetic ordering is concomitant with a transition to an insulating state. Here, we find that for low electron doping, the magnetic order on the nickel site is preserved while electronically a new metallic phase is induced. We show that this metallic phase has a Fermi surface that is mostly gapped by an electronic reconstruction driven by the bond disproportionation. Furthermore, we demonstrate the ability to write to and read from the spin structure via a large zero-field planar Hall effect. Our results expand the already rich phase diagram of the rare-earth nickelates and may enable spintronics applications in this family of correlated oxides.