Researchers from The Chinese University of Hong Kong and Stanford University/SLAC National Accelerator Laboratory have identified the origin of diagonal spin stripes within the bilayer nickelate La₃Ni₂O₇, a material recently recognized as a high-temperature superconductor. The work demonstrates that (π/2,π/2) spin stripe order arises in their model with sizable Hund’s coupling J_H from a hidden quasi-one dimensionality and persists over a range of electron concentrations. The results pinpoint interlayer coupling J_⊥ as a key factor enhancing pairing tendencies when the interlayer antiferromagnetic coupling J_⊥ becomes sufficiently large, providing a microscopic understanding of magnetic order within La₃Ni₂O₇.
Bilayer Nickelate La₃Ni₂O₇ and High-Temperature Superconductivity
La₃Ni₂O₇, a bilayer nickelate material, recently demonstrated superconductivity with transition temperatures as high as 80K under high hydrostatic pressure, a finding that has spurred intense investigation into its unusual magnetic properties. The emergence of this high-temperature superconductivity is coupled with the formation of spin stripe order at ambient pressure, prompting scientists to explore the microscopic origins of this behavior.
Understanding the interplay between magnetism and superconductivity in this material is crucial, given that superconductivity appears only under specific conditions of pressure or strain. A key aspect of the material’s structure is the arrangement of nickel atoms within layers, forming a square lattice with buckled oxygen atoms, a configuration that differs depending on pressure.
Researchers have modeled this system with a focus on nearest neighbor interactions between orbitals, utilizing varying hopping integrals, designated as t for strong bonds and t’ for weaker bonds, to reflect the Amam structure present at ambient pressure. This approach, similar to that used in previous studies of pressure-induced superconductivity, allows for a detailed examination of how structural changes impact electronic behavior.
The model retains only the nearest neighbor hoppings, acknowledging that the structure at ambient pressure differs from the more symmetric I4/mmm structure observed under high pressure or compressive strain, where t’ equals t. Calculations utilizing this model reveal that the diagonal spin stripes observed in La₃Ni₂O₇ originate from a hidden quasi-one dimensionality within the material, and are sustained across a range of electron concentrations. These calculations demonstrate that the interlayer pattern arises from antiferromagnetic coupling, designated J⊥, which connects the layers and influences the magnetic order. The work provides a microscopic origin of the diagonal spin stripes and identifies Hund’s coupling and interlayer coupling J⊥ as key ingredients governing magnetic order and pairing tendencies in La₃Ni₂O₇.
Origin of (π/2,π/2) Spin Stripe Order at Ambient Pressure
La₃Ni₂O₇, a bilayer nickelate material, is attracting considerable attention due to its emergence as a high-temperature superconductor exhibiting unusual spin stripe order even at relatively high temperatures. Unlike many materials where these spin patterns, arrangements of aligned electron spins, appear only at temperatures near absolute zero, La₃Ni₂O₇ maintains this order up to approximately 150K, presenting a puzzle for condensed matter physicists. Understanding the formation of these spin stripes at elevated temperatures is crucial, as it may unlock pathways to designing new materials with enhanced superconducting properties.
Recent strain engineering of thin films confirmed superconductivity at 40K only under compressive strain, while the same (π/2,π/2) spin stripe order reappeared without it. This model accurately reflects the crystalline symmetry of La₃Ni₂O₇ and utilizes density matrix renormalization group calculations to demonstrate that the (π/2,π/2) spin stripe order originates from a hidden quasi-one dimensionality.
The calculations reveal this order arises when the material exhibits a sizable Hund’s coupling, , and remains stable across a range of electron concentrations. This suggests a robust mechanism for spin alignment within the material’s structure, independent of minor variations in its composition. The research team’s model also explores the behavior of La₃Ni₂O₇ under high pressure. In these more symmetric conditions, the calculations indicate enhanced interlayer pairing tendencies when the interlayer antiferromagnetic coupling, J⊥, becomes sufficiently strong.
This finding suggests a potential link between the spin stripe order and the emergence of superconductivity, as pairing is a key characteristic of superconducting materials. The model incorporates a hopping non-uniformity, t’, for the d(x²-y²) orbitals and approximates the d(z²) orbitals as local moments coupled only through J⊥. The significance of this work lies in its ability to pinpoint the specific interactions driving the observed spin order. Previous attempts to understand the (π/2,π/2) spin stripe order in La₃Ni₂O₇ have faced challenges, but this new model offers a detailed microscopic explanation.
Model Parameters: J_H, J_⊥, and Hopping Anisotropy (t’, t)
The approach to modeling the bilayer nickelate La₃Ni₂O₇ taken by researchers centers on accurately representing the material’s crystalline symmetry to understand its unusual magnetism at ambient pressure. This specific formulation allows for detailed investigation of the spin stripe order, patterns where electron spins align, that emerges in La₃Ni₂O₇, a key characteristic of this potential high-temperature superconductor.
The model’s flexibility extends to simulating high-pressure conditions, achieved by setting t’ equal to t, effectively representing the I4/mmm structure formed under compression or strain in thin film samples. This ability to adjust hopping parameters allows for a comparative analysis of the material’s behavior under varying conditions, revealing how structural changes influence magnetic order and potential superconductivity.
Investigations into a simplified scenario, where hopping on the weak bonds vanishes (t’ approaching zero), revealed a connection to a one-dimensional ‘zig-zag’ Kondo-Hubbard lattice, a configuration known to exhibit period-4 charge density waves at 1/4 filling. This simplification, while artificial, provides a foundation for understanding the complex interplay of interactions within the full model and highlights the importance of the Hund’s coupling, , in determining the ground state.
The researchers state that further analysis of interlayer singlet pair correlations suggests the possibility of s-wave pairing, though the presence of coupling between the d(x²-y²) and d(z²) orbitals could lead to more complex s±-wave pairing scenarios. The model’s ability to predict these pairing tendencies offers a pathway for future investigations into the superconducting properties of La₃Ni₂O₇ and related materials.
DMRG Calculations Reveal Quasi-One-Dimensionality Driving SSO
Density matrix renormalization group calculations have illuminated a surprising origin for the diagonal spin stripes observed in the high-temperature superconductor La₃Ni₂O₇, revealing a hidden quasi-one-dimensional behavior driving this magnetic order. This finding offers a crucial microscopic understanding of the material’s unusual magnetism at ambient pressure, a long-standing challenge in condensed matter physics. The research team employed a sophisticated computational framework to ensure accuracy, allowing for detailed investigation of the interplay between various parameters within a microscopic Hamiltonian designed to reflect the crystalline symmetry of La₃Ni₂O₇.
The model incorporates both the d(z²) and d(x²-y²) orbitals, acknowledging the structural differences induced by tetragonal elongation, which affects orbital filling even with strong interactions present. This approach distinguishes itself from simpler models by allowing for a focused analysis of single layers, before tackling the high-pressure regime.
The team found that increasing J⊥ strengthens the potential for s-wave pairing between layers, a phenomenon that could be central to the material’s superconducting properties. The researchers state that this observation suggests that manipulating interlayer interactions could be a viable pathway for optimizing superconductivity in similar nickelate materials. The DMRG calculations also explored the impact of varying the hopping parameter, t′, relative to t, to simulate different structural regimes.
When t′ equals t, representing a compressively strained thin film or bulk material under high pressure, the system retains tetragonal symmetry, allowing for a more straightforward analysis. In contrast, when t′ is significantly different from t, the calculations require inclusion of both orbitals in both layers, increasing computational complexity. The team’s analysis of spin-spin correlations confirmed the presence of both (π/2,π/2) and (π,0) spin stripe orders, demonstrating the model’s ability to accurately reproduce observed magnetic phases.
Pressure/Strain Effects on Pairing and Structural Symmetry
La₃Ni₂O₇, a bilayer nickelate material, challenges conventional understanding of magnetic order by exhibiting spin stripe patterns at unexpectedly high temperatures, a phenomenon that could accelerate the development of high-temperature superconductors. While many materials display these aligned electron spins, the persistence of this order at elevated temperatures in this nickelate is prompting a re-evaluation of the underlying mechanisms at play. Researchers are now focusing on the interplay between structural symmetry and electron behavior to unlock the secrets of this material’s unique properties.
This coupling, alongside the material’s electron concentration, sustains the spin stripe arrangement, providing a microscopic origin for the observed diagonal stripes. The calculations demonstrate that this order isn’t simply a static feature, but one that dynamically responds to changes in the material’s environment, particularly under pressure.
The team’s model faithfully reflects the crystalline symmetry of the system, aiming to address its unconventional magnetism at ambient pressure, and providing a foundation for understanding its behavior under more extreme conditions. Applying pressure to La₃Ni₂O₇ induces a structural transition, shifting the material from a distorted Amam structure at ambient pressure to a more symmetric Fmmm or even I4/mmm arrangement. This change, mirrored in strain-engineered thin films, impacts the hopping of electrons between nickel atoms.
At ambient pressure, the nickel sites are distorted, creating strong and weak bonds, represented by hopping parameters t and t’, respectively. However, when the material is compressed, either through external pressure or within a thin film, t’ approaches t, effectively creating a more uniform electronic landscape. This symmetry change is not merely structural; it has a direct impact on the potential for electron pairing, a crucial ingredient for superconductivity.
In this more symmetric, high-pressure regime, the interlayer antiferromagnetic coupling, J⊥, becomes increasingly important. Calculations show that when J⊥ is sufficiently large, the material exhibits enhanced tendencies for interlayer pairing.
👉 More information
🗞 Spin Stripes and Superconductivity in Bilayer Nickelates
✍️ Hao-Xin Wang et al.
🧠 DOI: http://link.aps.org/doi/10.1103/zqg5-gzbq
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