Infinite-Layer Nickelates

• Infinite-layer nickelates are transition metal oxides with square-planar NiO₂ layers separated by rare-earth ions, enabling high-temperature superconductivity upon proper doping.
• Synthesis involves topotactic reduction of perovskite precursors, where careful control of cation stoichiometry and epitaxial strain is critical for stabilizing the superconducting phase.
• Their unconventional superconductivity arises from a complex interplay of multiorbital correlations, self-doping, and 3D Fermi surface effects that promote antiferromagnetic spin fluctuations and Kondo-lattice behavior.

Infinite-layer nickelates are a recently established family of transition metal oxides with the nominal formula RNiO₂ (R = rare earth), notable for exhibiting high-temperature superconductivity upon appropriate carrier doping. Their structure consists of square-planar NiO₂ layers separated by rare-earth ion layers, reminiscent of the arrangement in cuprate superconductors. However, the interplay of electronic correlations, multiband character, structural instability, and strong rare-earth/transition-metal hybridization produces a manifold that is both structurally analogous and yet profoundly distinct from the cuprates. The physics of infinite-layer nickelates provides a new platform for exploring unconventional superconductivity and the interplay between Mottness, Hundness, and Kondo-lattice-like phenomena.

1. Crystal and Electronic Structure

Infinite-layer nickelates crystallize in a simple tetragonal structure (space group P4/mmm) at ambient conditions for early rare earths (La, Nd, Pr), where transition metal oxide layers (NiO₂) alternate with rare-earth layers (R). Formal valence counting gives Ni¹⁺ (3d⁹), nominally isoelectronic with Cu²⁺ in cuprates. However, detailed experiments and theory reveal several crucial distinctions:

• The Ni 3d₍ₓ²₋ᵧ²₎ orbital dominates the electronic states near the Fermi energy, with a planar character and a markedly reduced hybridization with O 2p orbitals compared to cuprates, reflected in the O K-edge XAS by the absence of a pre-edge peak (Hepting et al., 2019).
• In contrast to the electrically inert and insulating spacer layers in cuprates, the rare-earth layer in nickelates hosts a three-dimensional, weakly interacting metallic state primarily derived from rare-earth 5d orbitals. This 5d state produces itinerant conduction electrons and introduces a 3D Fermi surface pocket (Hepting et al., 2019, Krieger et al., 2022).
• The result is a dual-subsystem: a quasi–two-dimensional strongly correlated NiO₂ layer (effectively a half-filled single-band Hubbard layer under LDA+U, U ≃ 6 eV, showing Mott–Hubbard features) and a weakly interacting, three-dimensional rare-earth metallic band (Hepting et al., 2019).
• The hybridization between these two channels is weak but physically significant and leads to an effective Anderson– or Kondo–lattice–like model where the NiO₂ layers emulate the role of the correlated 4f lattice in heavy fermion systems (Hepting et al., 2019).
• The presence of small rare-earth 5d electron pockets and their coupling to the NiO₂ correlations are essential for many emergent phenomena, including carrier “self-doping” and unusual magnetic excitations.

2. Synthesis, Structural Instabilities, and Heteroepitaxy

The infinite-layer phase is not the thermodynamically favored oxide structure. Synthesis typically employs a topotactic soft-chemical reduction, starting from a perovskite RNiO₃ film and selectively removing apical oxygens to obtain square-planar NiO₂ units, often using CaH₂ or similar hydrogenous reagents in vacuum at 240–280°C (Gutiérrez-Llorente et al., 22 Jan 2024, Puphal et al., 2022).

• High-quality films require nearly ideal cation stoichiometry in the precursor; deviations (e.g., cation vacancies) propagate and degrade superconducting properties in the reduced phase (Gutiérrez-Llorente et al., 22 Jan 2024).
• Structural instabilities, especially O₄ square rotations, are pronounced in compounds with smaller R (smaller cationic radius), where A-B cation mismatch and steric effects destabilize the high-symmetry P4/mmm phase and favor Glazer-type tilt patterns (e.g., a⁻a⁻c⁺), sometimes leading to an orthorhombic Pbnm or I4/mcm structure (Álvarez et al., 2021, Xia et al., 2021, Zhang et al., 2022).
• Hydrogen intercalation during reduction is favorable from the standpoint of energetics for many RNiO₂ members, and results in further enhancement of octahedral rotations and a tendency toward a gapped state with high-spin Ni moments (Álvarez et al., 2021).
• For bulk polycrystalline samples reduced with CaH₂, extra hydrogen remains sequestered in grain boundaries or secondary phase precipitates, and is not statistically incorporated into the average crystal structure of the infinite-layer phase (Puphal et al., 2022).
• Heteroepitaxy, including choices of substrate and capping layer, significantly affect both phase stabilization and the distribution of oxygen vacancies and nickel valence states (Ortiz et al., 6 Feb 2025, Krieger et al., 2022). Soft x-ray reflectivity has revealed substantial valence modulation and apical oxygen disorder at interfaces, which can be partially mitigated through epitaxial engineering, profoundly impacting the formation of superconducting and metallic states.

3. Multiorbital, Electride-like, and Self-Doping Physics

The electronic structure of infinite-layer nickelates departs from the idealized single-band Mott physics long established in cuprates. Notably:

• The removal of apical oxygen leaves a “void” that behaves like an electride site—an interstitial location with an attractive potential for electrons and local s-symmetry “zeronium” orbital (Foyevtsova et al., 2022). A significant fraction of the electrons freed during reduction are not fully transferred to Ni, but partially occupy this zeronium s-orbital, which strongly hybridizes with Ni 3d₍₃z²₋r²₎, resulting in one-dimensional-like band dispersion along c and major out-of-plane electronic/magnetic coupling.
• The balance between planar Ni 3d₍ₓ²₋ᵧ²₎ states, out-of-plane Ni 3d₍₃z²₋r²₎–zeronium hybridizations, and rare-earth 5d pockets creates a strongly multiband, multiorbital scenario. This hybridization is at the core of the differences in Fermi surface topology, dimensionality, and the challenges for theoretical simulation (significant dependence on the DFT code and the form of the correlated orbital projector) (Foyevtsova et al., 2022).
• The self-doping effect, arising from the proximity of the rare-earth 5d, interstitial-s, and Ni-3d bands, leads to an inhomogeneous electronic landscape in which the parent infinite-layer compound is not a true Mott insulator, but a “self-doped” (partially filled) system with weak hybridization between two- and three-dimensional states (Hepting et al., 2019, Ortiz et al., 2021).

4. Correlated Magnetism, Spin Fluctuations, and Charge Order

The low-energy spin dynamics in infinite-layer nickelates are governed by both the strongly correlated 3d₍ₓ²₋ᵧ²₎ electrons (Mottness) and their coupling to rare-earth itinerant electrons:

• Resonant inelastic x-ray scattering (RIXS) experiments observe dispersive magnetic excitations with bandwidth ≈ 200 meV in undoped NdNiO₂, akin to magnons in 2D S=1/2 antiferromagnets, despite a lack of long-range order. Linear spin wave fits yield J₁ ≈ 64 meV, J₂ ≈ –10 meV, where the negative J₂ is a signature of longer-range RKKY-like interactions mediated by itinerant 5d electrons (Lu et al., 2021, Rossi et al., 2023).
• Upon doping, the spectral weight and energy of magnetic modes decrease while their damping increases, eventually leading to overdamped dynamics—a haLLMark of increased scattering due to mobile carriers. The persistence of robust, dispersive spin excitations strongly supports the presence of Mottness and localized correlations, but with significant damping from rare-earth conduction bands (Lu et al., 2021).
• Charge order develops via a unique, multi-band mechanism: a strong on-site Hubbard U on Ni-d states induces an electronic redistribution to adjacent conduction (Nd-d, interstitial-s) bands, forming a stripe-like Ni$^{1+}$–Ni$^{2+}$–Ni$^{1+}$ pattern (q ≈ (1/3,0,0)) stabilized by the competition between potential energy gain (double occupancy avoidance) and charge transfer/kinetic cost (Chen et al., 2022). While this is robust in some samples (e.g., LaNiO₂), it appears sensitive to substrate, strain, and rare-earth hybridization (Rossi et al., 2023).

5. Superconductivity: Pairing Symmetry, Mechanism, and Phase Diagram

Superconductivity in infinite-layer nickelates emerges upon doping with divalent cations (e.g., Sr), but its microscopic origin diverges in key respects from the cuprate paradigm:

• Measurements of the London penetration depth in La₀.₈Sr₀.₂NiO₂ and Pr₀.₈Sr₀.₂NiO₂ reveal a quadratic temperature dependence of the superfluid density at T ≪ T_c, indicative of nodal (likely d-wave) pairing under strong disorder (dirty limit) (Harvey et al., 2022). Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ exhibits anomalous behavior due to magnetic impurity scattering.
• Superconductivity is quasi–two-dimensional and highly anisotropic. High-quality La-based films display 2D features such as cusp-like T_c(θ) in angular-dependent transport and signatures of a Berezinskii-Kosterlitz-Thouless transition near T_c, validating the dominance of Ni 3d₍ₓ²₋ᵧ²₎ in pairing (Sun et al., 2022).
• The superconducting phase diagram is characterized by a dome at intermediate hole doping, flanked by weak-insulator states: Mottness at low doping (holes occupy d₍ₓ²₋ᵧ²₎, strong localization, as in cuprates) and Hundness at high doping (holes in d_{xy}, local high-spin configurations driven by Hund’s coupling, as in Hund’s metals) (Xie et al., 2021). The crossover between these regimes produces the superconducting region.
• Theory and experiment agree that, at low doping, pairing is d-wave and stabilized by strong antiferromagnetic spin fluctuations, but with increasing doping the multi-band coherence and Fermi surface reconstruction (especially with the emergence of d_{z²}–derived states or 3D van Hove singularities) can favor a nodal s_± state (Kreisel et al., 2022).
• Electron-phonon coupling, even after many-body correlation enhancement (GW), is insufficient to account for the high T_c observed, with calculated λ values unable to yield T_c above a few tenths of a Kelvin. Thus, superconductivity in these systems is preempted by an unconventional, likely magnetically mediated mechanism (Meier et al., 2023).

6. Fermi Surface Dimensionality, van Hove Singularities, and Enhancement of Superconductivity

A marked departure from the cuprate single-orbital 2D Fermiology is observed:

• Angle-resolved measurements and theoretical analyses establish a 3D Fermi surface for infinite-layer nickelates, with pronounced k_z dispersion induced by hybridization between Ni-d₍ₓ²₋ᵧ²₎ and effective s/zeronium orbitals (Xia et al., 26 Apr 2025).
• This 3D topology produces van Hove singularities (VHS) at specific k_z values, yielding a logarithmic divergence in the DOS and thereby amplifying antiferromagnetic spin fluctuations (Xia et al., 26 Apr 2025).
• A combined RPA and DMFT methodology confirms that stronger spin fluctuations—due to proximity to VHS in the 3D Fermi surface—substantially enhance superconducting pairing, as evidenced by a larger eigenvalue in the gap equation, relative to idealized 2D single-band cases. This effect persists for d₍ₓ²₋ᵧ²₎ pairing but is crucially tied to the multi-band, three-dimensionality of the electronic structure (Xia et al., 26 Apr 2025).

7. Interface Physics, Disorder, and Material Design

Infinite-layer nickelate superconductivity has so far been realized primarily in epitaxially stabilized thin films, where interface phenomena and disorder become controlling factors:

• The polar discontinuity between substrates (e.g., SrTiO₃) and RNiO₂ films is resolved by the formation of intermediate layers such as Nd(Ti,Ni)O₃, which modulate local charge, suppress two-dimensional electron gas formation, and ensure the correct electron filling in NiO₂ planes (Goodge et al., 2022).
• At the atomic scale, heteroepitaxy, capping layers, and choice of interlayer (e.g. SrTiO₃ vs. LaGaO₃ in superlattices) induce spatially localized modulations in valence and oxygen sublattice disorder, affecting the ratio and orientation of NiO₄ square-planar and NiO₅ square-pyramidal sites (Ortiz et al., 6 Feb 2025). The central nickelate layers in these heterostructures can recover a monovalent d⁹ configuration, essential for metallic and superconducting behavior, while remaining interface layers often host higher valence and greater disorder.
• The degree of disorder, especially in oxygen removal, and the spatial profile of valence modulation are shown to be essential for achieving superconductivity: a lower degree of basal/apical oxygen disorder correlates with metallic conductivity approaching the infinite-layer ideal (Ortiz et al., 6 Feb 2025).
• Epitaxial strain, controlled via substrate or film processing, is found to be a highly effective tuning knob, with compressive biaxial strain pushing the Fermi surface towards 2D character and enhancing in-plane AFM exchange, realizing a more 'cuprate-like' regime and permitting crossing of 3D to quasi-2D magnetic transitions (Zhang et al., 2022).

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These developments establish infinite-layer nickelates as a laboratory for strongly correlated electron phenomena at the intersection of Mott, Hund, Kondo-lattice, and electride physics, where superconductivity emerges from a complex interplay of orbital selectivity, structural instability, multi-band coherence, and dimensionality-tuned spin fluctuations. Continued advances in synthesis, interface engineering, and multiorbital modeling are expected to clarify the essential ingredients for high-T_c pairing in this distinct class of unconventional superconductors.