La₂NiO₄ under Pressure Exhibits Suppressed 0.4, 0.7 eV Stoner Parameter, Precluding Superconductivity

The pursuit of high-temperature superconductivity receives a significant contribution from research into nickelate materials, and a new study clarifies the electronic behaviour of lanthanum nickelate, La2NiO4, under extreme pressure. Shu-Hong Tang, Han-Yu Wang, and colleagues at Zhejiang University investigate how this material’s electronic structure evolves when squeezed, employing advanced computational modelling to reveal the interplay between electron interactions and potential superconductivity. Their calculations demonstrate that while La2NiO4 exhibits complex electronic behaviour, strong magnetic tendencies suppress the formation of superconducting states, and the type of electron pairing that does emerge is unlikely to support high-temperature superconductivity. This work provides crucial insight into the challenges of achieving superconductivity in nickelates and suggests that alternative strategies, such as chemical modification, may be necessary to unlock their full potential.

Scientists performed density functional theory calculations, enhanced by dynamical mean-field theory and random approximation methods, to investigate the material’s electronic structure and potential for superconductivity. These calculations reveal a complex interplay of electron correlations within the nickel orbitals, particularly the eg manifold, which dictates the material’s low-energy electronic properties. Detailed analysis of the band structures and Fermi surfaces served as a foundation for understanding the material’s behaviour.

Calculations consistently demonstrated that the low-energy physics near the Fermi surface is dominated by nickel 3d orbital contributions, specifically two hybridized bands formed by the eg orbitals. These bands generate an electron pocket near the Γ point, predominantly composed of d orbital character, and a hole pocket around the X point with dominant d and orbital contribution. Building upon these calculations, scientists constructed a two-orbital model focused on the nickel eg orbitals, allowing for a more focused investigation of electron hopping between orbitals and lattice sites. The values of the hopping parameters were systematically computed under varying pressures using computational software, providing a quantitative description of the electronic interactions within the material. Increasing pressure significantly broadens the bands near the Fermi level, enhancing intralayer hopping and strengthening hybridization between the orbitals, driven by amplified inter-site hopping. Notably, the emergence of a new electron pocket around the Z point, contributed by lanthanum states at pressures exceeding 25 GPa, acts as a self-doping mechanism, modifying the occupation of the nickel orbitals and influencing electron correlation effects.

Nickelate Superconductivity Proves More Complex Than Cuprates

This research provides a detailed analysis of the challenges in achieving high-temperature superconductivity in nickelate materials. The central question revolves around understanding why these materials, despite structural similarities to cuprate superconductors, exhibit significantly different electronic behaviour. The research suggests that achieving superconductivity in nickelates is far more complex than initially hoped, and simple models used for cuprates do not readily apply. It’s a nuanced argument that doesn’t dismiss the possibility of nickelate superconductivity, but significantly tempers expectations and points to the need for a deeper understanding of the electronic structure.

The research demonstrates that the standard mechanisms driving superconductivity in cuprates do not easily translate to nickelates. The localization of electrons within the nickel orbitals hinders the formation of the necessary electronic states. Detailed electronic structure calculations, using methods like density functional theory and dynamical mean-field theory, reveal these limitations. The effect of oxygen vacancies and doping on the electronic structure is explored, but these do not necessarily lead to the formation of superconducting states. The complex interplay of orbital ordering and its impact on the electronic properties can also suppress the formation of superconducting states.

The authors point out that many existing theoretical models used to describe superconductivity are inadequate for nickelates, requiring new approaches. The hybridization between different orbitals is crucial, and controlling this hybridization is key to potentially enhancing superconductivity. The research draws comparisons to other materials, like ruthenates, to provide context and highlight the unique challenges posed by nickelates. This research provides a more realistic assessment of the potential for high-temperature superconductivity in nickelates, cautioning against overoptimism and emphasizing the significant hurdles that need to be overcome. The findings provide valuable insights that can guide future research efforts and identify the key areas that need to be addressed to potentially achieve superconductivity in nickelates, contributing to a broader understanding of correlated electron systems and underscoring the importance of developing accurate theoretical models that can capture the complex behaviour of these materials.

Pressure Suppresses Superconductivity in Nickelate Layers

This research elucidates the electronic behaviour of monolayer nickelate La2NiO4 under pressure, revealing a complex interplay between electron correlation and magnetism. Calculations demonstrate that the material exhibits non-Fermi-liquid characteristics at low pressures, stemming from strong electron interactions within the nickel orbitals, a behaviour analogous to that observed in a related compound. Importantly, analysis of magnetic susceptibility indicates a robust tendency towards magnetic order, suppressing the possibility of superconductivity in its pristine state. While superconducting instabilities do emerge under pressure, their symmetry evolves in a manner that hinders high-temperature superconductivity.

At lower pressures, d-wave pairing dominates, but above 75 GPa, the system transitions to an s+g-wave symmetry, which incurs energetic penalties due to the high-angular-momentum component. The researchers conclude that the absence of superconductivity in La2NiO4 arises from the stability of its magnetic ground state and the unfavourable pairing symmetry induced by pressure. The authors acknowledge that achieving superconductivity in this material likely requires alternative strategies, such as chemical doping or the application of epitaxial strain, to suppress magnetism while preserving the necessary conditions for strong spin fluctuations. Future work could investigate how subtle changes in composition interact with other factors to tailor the superconducting properties of these nickelates.