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1. Nickelate Superconductivity without Rare‐Earth Magnetism: (La,Sr)NiO ₂

   https://doi.org/10.1002/adma.202104083  

   Osada, Motoki; Wang, Bai Yang; Goodge, Berit H.; Harvey, Shannon P.; Lee, Kyuho; Li, Danfeng; Kourkoutis, Lena F.; Hwang, Harold Y. (September 2021, Advanced Materials)

   Abstract The occurrence of unconventional superconductivity in cuprates has long motivated the search for manifestations in other layered transition metal oxides. Recently, superconductivity is found in infinite‐layer nickelate (Nd,Sr)NiO₂and (Pr,Sr)NiO₂thin films, formed by topotactic reduction from the perovskite precursor phase. A topic of much current interest is whether rare‐earth moments are essential for superconductivity in this system. In this study, it is found that with significant materials optimization, substantial portions of the La_1−xSr_xNiO₂phase diagram can enter the regime of coherent low‐temperature transport (x = 0.14 ‐ 0.20), with subsequent superconducting transitions and a maximum onset of ≈9 K atx = 0.20. Additionally, the unexpected indication of a superconducting ground state in undoped LaNiO₂is observed, which likely reflects the self‐doped nature of the electronic structure. Combining the results of (La/Pr/Nd)_1−xSr_xNiO₂reveals a generalized superconducting dome, characterized by systematic shifts in the unit cell volume and in the relative electron‐hole populations across the lanthanides. 

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2. Structural transitions, octahedral rotations, and electronic properties of A3Ni2O7 rare-earth nickelates under high pressure

   https://doi.org/10.1038/s41535-024-00648-0  

   Geisler, Benjamin; Hamlin, James_J; Stewart, Gregory_R; Hennig, Richard_G; Hirschfeld, P_J (April 2024, npj Quantum Materials)

   Abstract Motivated by the recent observation of superconductivity withT_c ~ 80 K in pressurized La₃Ni₂O₇¹, we explore the structural and electronic properties ofA₃Ni₂O₇bilayer nickelates (A = La-Lu, Y, Sc) as a function of pressure (0–150 GPa) from first principles including a Coulomb repulsion term. At ~ 20 GPa, we observe an orthorhombic-to-tetragonal transition in La₃Ni₂O₇at variance with x-ray diffraction data, which points to so-far unresolved complexities at the onset of superconductivity, e.g., charge doping by variations in the oxygen stoichiometry. We compile a structural phase diagram that establishes chemical and external pressure as distinct and counteracting control parameters. We find unexpected correlations betweenT_cand thein-planeNi-O-Ni bond angles for La₃Ni₂O₇. Moreover, two structural phases with significantc⁺octahedral rotations and in-plane bond disproportionations are uncovered forA = Nd-Lu, Y, Sc that exhibit a pressure-driven electronic reconstruction in the Nie_gmanifold. By disentangling the involvement of basal versus apical oxygen states at the Fermi surface, we identify Tb₃Ni₂O₇as an interesting candidate for superconductivity at ambient pressure. These results suggest a profound tunability of the structural and electronic phases in this novel materials class and are key for a fundamental understanding of the superconductivity mechanism. 

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3. Absence of 3a0 charge density wave order in the infinite-layer nickelate NdNiO2

   https://doi.org/10.1038/s41563-024-01797-0  

   Parzyck, C T; Gupta, N K; Wu, Y; Anil, V; Bhatt, L; Bouliane, M; Gong, R; Gregory, B Z; Luo, A; Sutarto, R; et al (April 2024, Nature Materials)

   Abstract A hallmark of many unconventional superconductors is the presence of many-body interactions that give rise to broken-symmetry states intertwined with superconductivity. Recent resonant soft X-ray scattering experiments report commensurate 3a₀charge density wave order in infinite-layer nickelates, which has important implications regarding the universal interplay between charge order and superconductivity in both cuprates and nickelates. Here we present X-ray scattering and spectroscopy measurements on a series of NdNiO_2+xsamples, which reveal that the signatures of charge density wave order are absent in fully reduced, single-phase NdNiO₂. The 3a₀superlattice peak instead originates from a partially reduced impurity phase where excess apical oxygens form ordered rows with three-unit-cell periodicity. The absence of any observable charge density wave order in NdNiO₂highlights a crucial difference between the phase diagrams of cuprate and nickelate superconductors. 

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4. Local structure of Sr ₂ CuO _3.3 , a 95 K cuprate superconductor without CuO ₂ planes

   https://doi.org/10.1073/pnas.1918890117  

   Conradson, Steven D.; Geballe, Theodore H.; Jin, Changqing; Cao, Lipeng; Baldinozzi, Gianguido; Jiang, Jack M.; Latimer, Matthew J.; Mueller, Oliver (February 2020, Proceedings of the National Academy of Sciences)

   The local structure of the highly “overdoped” 95 K superconductor Sr₂CuO_3.3determined by Cu K X-ray absorption fine structure (XAFS) at 62 K in magnetically oriented samples shows that 1) the magnetization is perpendicular to thecaxis; 2) at these levels of precision the Cu sublattice is tetragonal in agreement with the crystal structure; the O sublattice has 3) continuous -Cu-O- chains that orient perpendicular to an applied magnetic field; 4) approximately half-filled -Cu-O- chains that orient parallel to this field; 5) a substantial number of apical O vacancies; 6) O ions at some apical positions with expanded Cu-O distances; and 7) interstitial positions that imply highly displaced Sr ions. These results contradict the universally accepted features of cuprates that require intact CuO₂planes, magnetization along thecaxis, and a termination of the superconductivity when the excess charge on the CuO₂Cu ions exceeds 0.27. These radical differences in charge and structure demonstrate that this compound constitutes a separate class of Cu-O–based superconductors in which the superconductivity originates in a different, more complicated structural unit than CuO₂planes while retaining exceptionally high transition temperatures. 

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5. Superconductivity in the Parent Infinite-Layer Nickelate ${\mathrm{NdNiO}}_2$

   https://doi.org/10.1103/PhysRevX.15.021048  

   Parzyck, C T; Wu, Y; Bhatt, L; Kang, M; Arthur, Z; Pedersen, T M; Sutarto, R; Fan, S; Pelliciari, J; Bisogni, V; et al (May 2025, Physical Review X)

   We report evidence for superconductivity with onset temperatures up to 11 K in thin films of the infinite-layer nickelate parent compound ${\mathrm{NdNiO}}_2$. A combination of oxide molecular beam epitaxy and atomic hydrogen reduction yields samples with high crystallinity and low residual resistivities, a substantial fraction of which exhibit superconducting transitions. We survey a large series of samples with a variety of techniques, including electrical transport, scanning transmission electron microscopy, x-ray absorption spectroscopy, and resonant inelastic x-ray scattering, to investigate the possible origins of superconductivity. We propose that superconductivity could be intrinsic to the undoped infinite-layer nickelates but suppressed by disorder due to a possibly sign-changing order parameter, a finding which would necessitate a reconsideration of the nickelate phase diagram. Another possible hypothesis is that the parent materials can be hole doped from randomly dispersed apical oxygen atoms, which would suggest an alternative pathway for achieving superconductivity. Published by the American Physical Society2025 

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