---
title: Nickelate Superconductivity
url: https://www.emergentmind.com/topics/nickelate-superconductivity
type: topic
---

# Nickelate Superconductivity

Nickelate superconductivity refers to the phenomenon of zero electrical resistance and expulsion of magnetic flux in layered nickel oxide compounds, structurally and electronically analogous to the high-Tc cuprate superconductors but displaying key distinctions in electronic structure, synthesis, and pairing mechanisms. The field has rapidly expanded since the 2019 discovery of superconductivity in hole-doped infinite-layer nickelates, now encompassing single- and multi-layered Ruddlesden-Popper phases, thin films, and pressurized bulk crystals, with transition temperatures (Tc) ranging from below 20 K to above 90 K in select bilayer systems.

## 1. Electronic Structure and Minimal Hamiltonians

The electronic structure of superconducting nickelates is governed by a dual character comprising strongly correlated, quasi–two-dimensional (2D) NiO₂ layers and weakly correlated, three-dimensional (3D) rare-earth (R) 5d metallic spacer layers. The Ni site is typically in a $d^9$ or $d^{8.8}$ configuration, with the $3d_{x^2-y^2}$ orbital forming the principal low-energy band. In contrast to cuprates—where the charge-transfer insulator scenario is paradigmatic—rare-earth nickelates feature an “oxide-intermetallic” state, as insulating spacer layers are replaced by 3D metallic R 5d bands [1909.02678][1909.07688][2002.12230].

A representative effective Hamiltonian is:
\[
H = \sum_k \left( 
  \varepsilon_k^R c_k^\dagger c_k +
  \varepsilon_k^{Ni} d_k^\dagger d_k
\right) + U \sum_i n_{i,\uparrow}^{Ni} n_{i,\downarrow}^{Ni}
+ \sum_k \left( V_k c_k^\dagger d_k + \text{h.c.} \right)
\]
with $c_k^\dagger$ ($d_k^\dagger$) creating an electron in the rare-earth 5d (Ni $3d_{x^2-y^2}$) band, and $V_k$ representing weak interband hybridization.

This leads to a Kondo- or Anderson-lattice–like description, but with a 2D correlated Ni layer coupled to a 3D metallic fluid, rather than the 4f/5d system typical of heavy fermions.

## 2. Phases, Superconducting Pairing, and Contrasts with Cuprates

Nickelate superconductors display several critical differences and similarities with cuprates:
- **Cuprates:** Exhibit a Mott or charge-transfer insulating parent phase with insulating spacer layers. Superconductivity arises via hole doping that disrupts robust antiferromagnetism.
- **Nickelates:** The parent state is metallic due to rare-earth 5d conduction. The NiO₂ planes are close in structure to CuO₂ planes, and the $3d_{x^2-y^2}$ orbital is half-filled, but the system is not a true Mott insulator. The rare-earth spacer hybridizes weakly with Ni 3d, generating small 3D Fermi surface pockets [1909.02678][2509.08386].

Pairing is believed to be unconventional—governed by strong repulsive interactions—leading to a sign-changing (likely $d$-wave) order parameter, though alternative or coexisting $s_{\pm}$ and $d+is$ pairings may emerge in multiorbital models depending on hybridization and Coulomb terms [2310.12250]. In pressurized and multilayer nickelates (e.g., Ruddlesden-Popper phases), antiferromagnetic superexchange between $3d_{x^2-y^2}$ and $3d_{z^2}$ orbitals (enhanced by interlayer hopping and Hund's coupling) is implicated as the primary pairing interaction [2311.07423][2003.09132].

## 3. Material Families, Phase Diagrams, and Tc Range

Nickelate superconductors now span several crystallographic series:
- **Infinite-layer (112) series:** $R$NiO$_2$ (R = La, Pr, Nd) and their doped derivatives. Ca- and Sr-doped films (e.g., $Nd_{0.8}Sr_{0.2}NiO_2$, $La_{1-x}Ca_xNiO_2$) exhibit superconducting domes with $T_c$ ranging 10–20 K, but onset above 50 K has been reported in optimized thin films [2105.13492][2502.17892].
- **Bilayer (327) series:** $La_3Ni_2O_7$ and Sm-substituted variants, showing bulk superconductivity at $T_c$ up to 92 K (onset) and 73 K (zero resistance) under pressures of 20 GPa [2501.14584]. High-pressure tuning and chemical "pressure" (via rare-earth substitution) are essential for stabilizing the high-Tc phase and optimizing the Ni–O–Ni bond geometry.
- **Trilayer (4310) series:** $La_4Ni_3O_{10}$ and $Pr_4Ni_3O_{10}$ demonstrate bulks superconducting transitions at 25–40 K under high pressure, with Pr substitution raising $T_c$ higher than La due to internal chemical pressure [2501.17709].
- **Multilayer and hybrid phases:** Quintuple-layer compounds (e.g., $Nd_6Ni_5O_{12}$) achieve a cuprate-like $d^{8.8}$ filling and show superconductivity near 13–15 K without chemical doping [2109.09726]. Other hybrid phases with various layer stacking and oxygen stoichiometry have also been explored.

A universal feature across all families is a dome-shaped $T_c$ vs. doping or pressure phase diagram, reminiscent of cuprates. High-pressure or strain engineering of the lattice drives structural transitions (often monoclinic to tetragonal) that flatten NiO$_2$ planes and tune the electronic structure for optimal superconductivity [2501.14584][2407.16495].

## 4. Normal State, Transport Properties, and Multiband Effects

Nickelate superconductors show a correlated normal-state regime analogous to cuprates but with unique features:
- In the underdoped regime, $\rho(T)$ exhibits a low-temperature upturn (logarithmic or insulating), linked to strong correlations and possible Kondo-like scattering between Ni $3d$ local moments and rare-earth $5d$ conduction electrons [1909.07688][2203.02580].
- At optimal doping/pressure, strange-metal, linear-in-$T$ resistivity is universally observed, with slope ($\sim$11 m$\Omega$·cm/K) comparable to cuprates [2203.02580].
- In the overdoped or highly pressurized regime, resistivity exhibits conventional Fermi-liquid T$^2$ behavior.

Multiband effects are crucial: both the $3d_{x^2-y^2}$ and the rare-earth $5d$ bands cross the Fermi level, though weakly hybridized. The Hall coefficient $R_H$ displays nontrivial doping and temperature dependences, often undergoing sign-changing transitions at low temperatures linked to multiband carrier dynamics. In La-based nickelates, the Hall sign-change temperature is pinned near 35 K, while in Nd-/Pr-based systems it shifts with doping [2105.13492].

## 5. Synthesis, Structural Control, and Challenges

Synthesis of high-quality nickelate superconductors is a principal technical bottleneck. Critical strategies include:
- **Topotactic reduction** of a perovskite precursor using a reducing agent (commonly CaH$_2$) to obtain the infinite-layer phase.
- **Flux growth**, enabling ambient-pressure synthesis of high-purity bilayer nickelate single crystals with sizes up to 220 μm and excellent compositional homogeneity [2501.14584].
- **Strain/pressure tuning** through both bulk high-pressure apparatus and thin-film epitaxy on lattice-mismatched substrates.
- **Chemical pressure** via systematic rare-earth substitution, effectively compressing the Ni–O bond network and modifying interlayer distances [2501.14584][2501.17709].

Disorder and oxygen vacancies have complex roles. Oxygen vacancies (especially apical O) can drastically reconstruct the band structure, diminishing the Ni $d_{z^2}$ involvement at $E_F$ and suppressing superconductivity, as shown for $La_3Ni_2O_7$; Ce-based analogs with higher oxygen vacancy formation energies may better avoid this problem [2312.01271].

Persistent challenges include stabilizing the elusive Ni$^{1+}$ oxidation state, minimizing structural defects (e.g., Ruddlesden-Popper faults), achieving uniform stoichiometry, and suppressing competing density-wave states.

## 6. Theoretical Models and Mechanisms

Several theoretical approaches address nickelate superconductivity:
- **Single-band Hubbard model:** Appropriate for infinite-layer nickelates near optimal doping, once the weakly hybridized rare-earth $5d$ bands are treated as a carrier reservoir. Dynamical vertex approximation and DMFT yield $T_c$ domes and phase diagrams consistent with experiment [2002.12230][2205.00239].
- **Multiorbital/bilayer/t–J models:** Required for multilayer nickelates and systems with significant $d_{z^2}$ admixture. In trilayer phases, strong superexchange between $d_{x^2-y^2}$ and $d_{z^2}$ orbitals is crucial, with pairing gaps and $T_c$ values greatly enhanced compared to single-layer systems [2003.09132][2311.07423].
- **Competing order analysis:** Pressure suppresses density wave order, especially in $La_4Ni_3O_{10}$, and induces superconductivity—a feature paralleling but also departing from cuprate trends [2311.07423].

Multiorbital calculations reveal $d_{x^2-y^2}$ pairing is robust at low hybridization but may be suppressed by strong $3d$–$5d$ hybridization, giving rise to $s_{\pm}$ or $d+is$ states under specific conditions [2310.12250]. The primary pairing “glue” is identified as antiferromagnetic superexchange, though Anderson/Kondo-lattice physics arising from hybridization with metallic rare-earth layers alters the mechanism relative to the cuprates [1909.02678].

## 7. Outlook and Open Problems

Recent advances have demonstrated $T_c$ values surpassing the boiling point of liquid nitrogen in bilayer nickelates under pressure (onset >90 K) [2501.14584], diode and paramagnetic-Meissner phenomena in thin films [2502.17892], and bulk superconductivity with robust volume fractions in trilayer systems [2501.17709]. However, there remain outstanding questions:
- The true pairing symmetry is debated, complicated by possible multiband and multi-gap states. Experimental confirmation via phase-sensitive probes is needed.
- The influence of disorder, especially oxygen non-stoichiometry and apical O vacancy formation, is determinative yet still not fully controlled or understood.
- The relationship to cuprate superconductivity is subtle: while key features (optimum $d_{x^2-y^2}$ filling, $T_c$ dome, strange-metal transport) are echoed, the electronic starting points and pairing mechanisms exhibit both overlap and distinction—especially due to the persistent involvement of rare-earth 5d states and multiorbital physics.
- Implementation of advanced synthesis (direct low-T routes avoiding reduction disorder), targeted chemical substitutions (including 4d/5d analogs), and improved structural control are poised to further clarify the role of electronic structure, dimensionality, and disorder.

A plausible implication is that the field is converging toward a unifying framework encompassing both cuprates and nickelates in the wider context of correlated oxide superconductivity, centering on the optimization of $d_{x^2-y^2}$ states via layer and chemical control—while also recognizing essential multi-orbital and hybridization-induced distinctions that will inform future materials design and theoretical models [2407.16495][2509.08386].

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