Biaxial Strain Reshapes Bilayer Nickelates Through a Distinct Electronic Pathway

By Akshatha ChandrashekarReviewed by Susha Cheriyedath, M.Sc.Aug 6 2026

Compression tunes bilayer nickelates without forcing the high-pressure phase, revealing an alternative route to the orbital and band conditions linked to superconductivity.

Paper: Structural and Electronic Evolution of Bilayer Nickelates Under Biaxial Strain. Image credit: AI-generated conceptual image created using ChatGPT/OpenAI  

A recent study published in the journal Communications Physics investigates how biaxial strain modifies the structural and electronic properties of bilayer rare-earth nickelates. Researchers examined how compressive and tensile strain influence atomic structure, orbital interactions, and band evolution across the RE₃Ni₂O₇ (RE = rare earth) series. These findings provide new insights into strain engineering as a strategy for tuning properties potentially relevant to superconductivity in nickelate thin films.

Understanding Strain Engineering in Bilayer Nickelates

The discovery of superconductivity in bilayer nickelates has renewed interest in these materials because they share important structural and electronic features with high-temperature cuprate superconductors. Initial experiments observed superconductivity in La₃Ni₂O₇ under high pressure, while more recent studies demonstrated that epitaxial compressive strain could stabilize superconductivity in thin films at ambient pressure. These findings position strain engineering as a more accessible experimental approach than high-pressure methods, although strain and pressure do not necessarily produce equivalent electronic states.

Previous studies focused on La₃Ni₂O₇ and often examined higher-symmetry crystal structures that differ from the low-symmetry phase relevant to strained thin films. It also remains unclear whether biaxial strain modifies the electronic structure through the same mechanisms as hydrostatic pressure or follows a distinct pathway. To address this gap, the researchers systematically investigated structural trends across the broader RE₃Ni₂O₇ family and performed detailed electronic analyses for RE = La, Pr, and Nd.

In this study, researchers examined how strain influences crystal structure, orbital hybridization, electronic bands, crystal-field splitting, and electron hopping. By comparing these changes with previously reported high-pressure behavior, the analysis clarifies how epitaxial strain can tailor the low-energy properties of bilayer nickelates through a pathway distinct from pressure-driven evolution.

Computationally Investigating Strain-Induced Electronic Evolution

The researchers investigated the effects of biaxial strain using first-principles density functional theory (DFT) calculations with on-site Hubbard corrections (DFT+U) to account for electron correlation in nickel 3d orbitals. They modeled non-magnetic structures in the experimentally relevant low-symmetry orthorhombic Amam phase of bilayer nickelates. The analysis included La₃Ni₂O₇, Pr₃Ni₂O₇, and Nd₃Ni₂O₇ to evaluate how different rare-earth elements influence the strain response.

The team applied biaxial strains ranging from compressive to tensile conditions while allowing the remaining structural parameters to relax. They monitored changes in lattice constants, Ni–O bond lengths, NiO₆ octahedral distortions, and the spacing between adjacent nickel-oxide layers. These structural variations were then linked to changes in the calculated band structure.

The researchers further analyzed the electronic properties using Wannier downfolding, which converts DFT results into an effective tight-binding model. This method enabled them to calculate orbital energies, crystal-field splitting, and electron hopping parameters that govern orbital-dependent electronic coupling. Finally, they compared the strain-dependent band structure with previously reported high-pressure results to determine whether the two approaches modify bilayer nickelates through similar or fundamentally different microscopic mechanisms.

Compressive Strain Reshaped Electronic Coupling Through Distinct Mechanisms

The calculations showed that biaxial strain substantially altered both the crystal structure and electronic properties of bilayer nickelates. Compressive strain reduced the in-plane lattice constants while expanding the out-of-plane lattice parameter, producing noticeable distortions of the NiO₆ octahedra. It also modified Ni–O bond lengths, bond angles, and the spacing between adjacent nickel-oxide layers. At moderate compression, however, the separation within the nickel-oxide bilayer remained relatively stable, with much of the out-of-plane expansion occurring through outer apical bonds and regions outside the bilayer.

The calculations also showed that moderate compression did not yield the high-symmetry structure observed in pressurized nickelates. In La₃Ni₂O₇, 2.5% compression increased the apical Ni–O–Ni bond angle from 168.5° to 171.1°, but the angle did not reach 180°, and the material remained in the Amam phase. At higher compression, the response was not uniformly monotonic because the apical angle in La- and Pr-based compounds began moving away from 180°.

These structural modifications directly influenced the electronic structure. Compressive strain increased the overlap between Ni 3d_z² and apical oxygen 2p_z orbitals, strengthening their hybridization and modifying the bands near the Fermi level. Under 2.5% compression, the oxygen contribution to selected bonding and antibonding bands near the Fermi level increased by approximately 10%. Wannier analysis also revealed changes in the crystal-field splitting and electron-hopping parameters, indicating an orbital-dependent redistribution of electronic coupling within the nickel-oxide framework.

Under 2.5% compressive strain, interlayer Ni 3d_z² hopping and in-plane Ni 3d_x²−y² hopping increased, while in-plane Ni 3d_z² hopping decreased. Thus, compression strengthened some interlayer and in-plane pathways while weakening another in-plane pathway. These changes made the 3d_x²−y² bands more dispersive while flattening the 3d_z² bands. The crystal-field splitting between these orbitals also increased substantially, shifting the 3d_z² bands downward and placing them entirely below the Fermi level.

The comparison revealed that biaxial strain and hydrostatic pressure influence bilayer nickelates through different microscopic mechanisms. Under high pressure, previous calculations found that the 3d_z² bands become more dispersive, leading to a new hole pocket near the Fermi level. Under compressive strain, the bonding 3d_z² bands instead moved to lower energy and farther below the Fermi level. Rather than simply reproducing the effects of high pressure, epitaxial strain modifies the band structure by independently tuning in-plane and out-of-plane atomic interactions.

The researchers also identified consistent trends across the rare-earth series. Replacing lanthanum with smaller rare-earth elements systematically altered the structural response to strain and the resulting electronic coupling. While La₃Ni₂O₇, Pr₃Ni₂O₇, and Nd₃Ni₂O₇ showed similar band-structure trends, the structural response varied across the wider lanthanide series. These findings suggest that combining epitaxial strain with rare-earth substitution could provide complementary variables for tailoring the properties of bilayer nickelates and investigating conditions associated with superconducting behavior.

Guiding the Study of Superconducting Nickelates

The study demonstrates that biaxial strain can substantially tune the structural and electronic properties of bilayer nickelates. Epitaxial strain modifies orbital hybridization and electron hopping via distinct structural changes, creating an electronic environment distinct from that produced by hydrostatic pressure. This difference makes strain engineering an independent strategy for examining microscopic interactions that may contribute to superconductivity in nickelate thin films.

The results also highlight the combined influence of crystal structure and chemical composition. The strain response and electronic coupling varied systematically with the size of the rare-earth element. This interplay between strain and composition offers additional opportunities to control the microscopic interactions that may influence superconducting behavior.

The calculations further suggest that a 3d_z²-derived band need not cross the Fermi level for superconductivity to emerge. This interpretation is consistent with recent photoemission measurements of strained superconducting nickelate films, although the calculations predicted a larger downward shift of the 3d_z² bands than was observed experimentally. The study did not directly calculate superconducting transition temperatures or pairing strength, so the relationship between these predicted band changes and superconductivity remains to be established.

The researchers note that further experimental studies are needed to validate the predicted orbital and band changes and establish their relationship with superconductivity. Future work should use many-body calculations to investigate magnetic susceptibility, superconducting pairing, and charge and spin dynamics, while experiments control oxygen concentration and phase purity. These studies will be needed to distinguish intrinsic strain-induced effects from extrinsic influences such as oxygen vacancies and structural imperfections.

Overall, the first-principles calculations and Wannier analysis provide a microscopic framework for future theoretical and experimental studies of strained nickelate superconductors.

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