---
title: Nickelate Superconductors
url: https://www.emergentmind.com/topics/nickelate-superconductors
type: topic
---

# Nickelate Superconductors

Nickelate superconductors designate a family of layered nickel oxides that exhibit unconventional superconductivity upon carrier doping or under high pressure. These materials include both infinite-layer compounds—typified by RNiO₂ (R = La, Nd, Pr)—and higher-order Ruddlesden–Popper (RP) phases such as bilayer (La₃Ni₂O₇) and trilayer (La₄Ni₃O₁₀) systems. Nickelate superconductors are of immense interest due to their formal and structural analogy to the high-Tc cuprates, but they also display distinct electronic correlations, multiband physics, and a complex interplay of charge, spin, orbital, and lattice degrees of freedom. Their phase diagrams encompass strange metallicity, density-wave instabilities, and high-pressure-induced superconducting domes. This article surveys the crystallography, experimental discovery, electronic structure, pairing mechanisms, role of defects and disorder, and outstanding challenges for this new class of superconductors.

## 1. Crystal Structures and Synthesis Pathways

Nickelate superconductors crystallize in layered phases structurally analogous to the cuprates but differ in Ni–O coordination, dimensionality, and block-layer chemistry. The principal members include:

- **Infinite-layer nickelates (RNiO₂):** Derived by topotactic reduction of perovskite RNiO₃, these compounds feature square-planar NiO₂ layers separated by rare-earth layers without apical oxygen. Thin films are typically stabilized on SrTiO₃(001) substrates via pulsed-laser deposition and subsequent reduction in CaH₂ or NaH. Superconductivity is observed in films with doping by Sr (or Ca) in a regime x ≈ 0.1–0.3 [2112.06765, 2509.08386].
- **Ruddlesden–Popper bilayer and trilayer nickelates (La₃Ni₂O₇ and La₄Ni₃O₁₀):** These phases, synthesized as bulk single crystals or strained thin films, feature n = 2 or 3 contiguous NiO₂ planes separated by LaO rock-salt slabs, alternating with octahedral or planar NiO₆ units. Under high pressure (P ≳ 9–15 GPa), both adopt a higher symmetry (Fmmm or I4/mmm) favoring superconductivity [2311.07423, 2403.05012, 2509.08386].
- **Key synthetic challenges:** The infinite-layer phase requires fine control over film thickness (≤10 nm), reduction conditions, and substrate-induced strain to avoid secondary RP faults. Multilayer nickelates demand high-pressure synthesis to access the HP superconducting structures; ambient-pressure thin-film phases necessitate stabilization by epitaxial strain or defect engineering. Controlling the concentration and distribution of oxygen vacancies and topotactic hydrogen is crucial for attaining homogeneous electronic properties.

## 2. Electronic Structure and Normal State Behavior

The low-energy physics is dominated by Ni 3d electrons with significant orbital selectivity, multiband effects, and correlations:

- **Single- and multi-band character:** Infinite-layer nickelates realize a Ni 3dₓ²₋ᵧ²-derived, quasi-two-dimensional Fermi surface, complemented by self-doped rare-earth 5d electron pockets at the BZ corners. RP bilayer and trilayer phases exhibit additional symmetry-derived bands (bonding/antibonding) from the coupling of Ni orbitals in adjacent layers [2012.02764, 1909.02678, 2509.08386].
- **Non-Fermi liquid transport:** Near optimal doping, the normal state shows linear-in-T resistivity extending to high temperatures (A₁ ≈ 6–11 μΩ·cm·K⁻¹), closely paralleling the "strange metal" regime in cuprates. Underdoped samples exhibit low-T upturns, attributed to charge-density wave (CDW) or localization effects, while overdoped samples display quadratic (T²) resistivity [2203.02580]. No hard Mott gap or AFM order is observed in undoped nickelates, contrasting with cuprates.
- **Correlation regime:** Spectroscopy and DFT+DMFT place nickelates primarily in the Mott-Hubbard regime (U ≈ 3–4 eV, J_H ≈ 0.7 eV), with less O 2p hybridization than cuprates. Quasiparticle mass renormalization and moderate Hund’s coupling induce strong orbital selectivity and, at strong doping, emergent multiorbital physics with possible spin-freezing crossovers [1910.00473, 2012.02764].

## 3. Superconducting Phase Diagrams and Experimental Phenomena

Superconductivity in nickelates displays rich phase behavior as a function of doping, pressure, and structural control:

| System                | Max Tc (K) | Superconducting Dome    | Ambient/Pressure      | Notes                                       |
|-----------------------|------------|-------------------------|----------------------|---------------------------------------------|
| RNiO₂ (112 films)     | ~15        | x ≈ 0.10–0.28           | Thin film, AP / P    | Bulk SC not reported; thin-film limited     |
| La₃Ni₂O₇ (327, HP)    | ~80        | P ≳ 9–20 GPa            | Bulk, Pressure       | Full diamagnetism, no AFM order            |
| La₄Ni₃O₁₀ (43(10), HP)| ~25        | P ≳ 20–25 GPa           | Bulk, Pressure       | Coincides with suppression of DW order      |

*AP: Ambient Pressure; P: Pressure-induced; DW: Density Wave*

Critical observations include:

- **Superconducting domes:** In both infinite-layer and RP systems, superconductivity appears within a dome-shaped region as a function of hole doping (infinite-layer) or pressure (RP phases), with maximal T_c at intermediate carrier concentrations or near the suppression of competing order [2509.08386, 2311.07423].
- **Competition with density waves:** RP nickelates at ambient pressure manifest CDW/SDW transitions (e.g., La₄Ni₃O₁₀, T_DW ∼ 132 K) that are progressively suppressed by pressure, with superconductivity emerging only after the density-wave order collapses [2311.07423, 2403.05012].
- **Bulk nature of superconductivity:** Meissner effect measurements in thin films and pressure-induced diamagnetism in RP phases confirm bulk superconducting states, excluding significant interface or filamentary contributions [2112.06765, 2311.07423].
- **3D anisotropy in thin films:** Despite thin-film geometry, upper critical field and coherence length analyses point to intrinsically three-dimensional superconductivity, with temperature-dependent anisotropy γ_ξ(T) and GL coherence lengths in the few-nm range [2305.05593].

## 4. Microscopic Pairing Mechanisms and Order Parameters

Theoretical and spectroscopic analyses converge on an unconventional, fluctuation-mediated superconducting state:

- **Spin-fluctuation-driven d-wave pairing:** The dominant mechanism across all nickelate families is attributed to spin fluctuations arising from strong on-site U and AFM superexchange (J_AF = 4t²/U). Weak electron-phonon coupling (λ ≪ 0.2) is insufficient for high-Tc [2012.02764, 1910.05757, 2205.00239]. In infinite-layer systems and inner layers of RP nickelates, the superconducting gap has dₓ²₋ᵧ² symmetry: Δ(k) ∼ cos kₓ − cos k_y [2112.06765, 2509.08386].
- **Multiband and orbital-selective effects:** In RP bilayers, bonding-antibonding and cross-orbital mixing between dx²–y² and dz² orbitals yield competing s±, d, or mixed states. The pairing interaction is further shaped by Hund’s coupling and the degree of orbital polarization [1910.00473, 2509.08386].
- **Role of electron-phonon coupling and collective modes:** Ultrafast dynamics reveal a coherent A_g phonon in La₄Ni₃O₁₀ absent in La₃Ni₂O₇. The extracted EPC constant λ increases from 0.05–0.07 (bilayer) to 0.12–0.16 (trilayer), marking enhanced lattice involvement but ruling out phonon-mediated superconductivity as the primary mechanism [2403.05012].
- **Pairing frustration and dimensionality:** Trilayer nickelates experience pairing frustration due to interlayer coupling, with the inner NiO₂ plane acting as a "pairing bottleneck" that suppresses Tc relative to the bilayer despite similar electronic structure [2311.07423]. Theoretical modeling shows superconductivity is maximized when density-wave order is fully suppressed yet nodal (outer-layer) d-wave gaps are maintained.

## 5. Defects, Disorder, and the Role of Topotactic Hydrogen

Variability in superconducting properties across samples is strongly linked to atomic-scale disorder, notably oxygen vacancies and topotactic hydrogen (H), both examined in experiment and theory:

- **Oxygen vacancies:** In La₃Ni₂O₇, apical oxygen vacancies induce a strong reduction of dz² spectral weight at E_F, reverse and enhance the intra-bilayer t_{z²}^{intra} and inter-orbital t_{z², x²−y²} hoppings, and suppress superconductivity by distorting the Fermi surface and orbital occupation [2312.01271]. Ce₃Ni₂O₇, with lower vacancy formation energy, may offer improved stability [2312.01271].
- **Topotactic hydrogen:** H atoms preferentially occupy apical sites, forming quasi-1D chains along the c-axis and chemically converting Ni¹⁺ (d⁹) to Ni²⁺ (d⁸, S = 1) in adjacent sites. This induces spatially inhomogeneous valence, causes charge disproportionation, drives 2D→3D magnetic crossovers, and facilitates exotic charge order. DFT+DMFT and impurity model calculations reveal that both low and high concentrations of H favor high-spin Ni²⁺ states with active d_{z²}, strongly suppressing superconductivity. Only at intermediate H concentration (~25%) does a single-band d_{x²−y²} sector survive, restoring a cuprate-analog superconducting channel [2208.11085, 2307.12020].
- **Spectroscopic fingerprints:** Topotactic H introduces flat, IR-active phonon modes (∼25 and 43 THz), serving as "smoking gun" evidence for H incorporation and chain formation [2204.10657, 2208.11085].

## 6. Future Directions and Open Challenges

Nickelate superconductors have catalyzed a new wave of theoretical and experimental investigations, yet present numerous unresolved issues:

- **Fermiology and pairing symmetry:** High-quality ARPES and phase-sensitive probes under various doping/pressure conditions are needed to unambiguously establish gap symmetry and multiband effects across RP members [2509.08386].
- **Disorder and homogeneity:** Quantitative control and measurement of O-vacancies and H content are crucial for correlating intrinsic superconducting properties with local structure [2312.01271, 2307.12020].
- **Ultrafast and mode-selective control:** Understanding and manipulation of density waves and superconductivity by coherent phonon excitation or mid-IR/THz pumping are emerging research avenues [2403.05012].
- **Materials design:** Selection of block layers for enhanced correlation tuning (e.g., Ce₃Ni₂O₇), exploration of further RP members, and integration of advanced simulation techniques (DFT+DMFT, DΓA, fRG) will drive discovery of new nickelate-based superconductors and higher Tc [2205.00239, 2509.08386].
- **Comparisons to cuprates and broader universality:** Despite key differences in orbital hybridization and electronic structure, the normal- and superconducting-state phenomenology of nickelates closely echoes the cuprates, including strange metallicity, quantum criticality, and Planckian-limited dissipation, suggesting potential universal organizing principles in correlated oxides [2203.02580, 2012.02764].

Advances in synthesis, characterization, and theoretical modeling continue to refine the understanding of the mechanisms underlying high-Tc superconductivity in nickelates, highlighting the role of orbital selectivity, dimensionality, and intertwined orders in shaping their emergent phases.

Source: https://www.emergentmind.com/topics/nickelate-superconductors