Researchers from The Chinese University of Hong Kong and Stanford University/SLAC National Accelerator Laboratory have pinpointed the microscopic origins of diagonal spin stripes within the bilayer nickelate La3Ni2O7. According to the study, a hidden quasi-one dimensionality drives the material’s unusual $(pi/2,pi/2)$ spin stripe order at ambient pressure, requiring sizable Hund’s coupling $J_H$ to persist across varying electron concentrations.
Unlocking the Mystery of La3Ni2O7 Magnetism
The Ruddlesden-Popper bilayer nickelate La3Ni2O7 has captured intense scientific interest as a high-temperature superconductor. Under high hydrostatic pressure, the material exhibits transition temperatures reaching approximately 80K, according to the research team. Yet, at ambient pressure, scientists observe an unusual spin stripe order with momentum $Q=(pi/2,pi/2)$ and an onset temperature around 150K.
Experimental techniques including resonant inelastic X-ray scattering, $mu$SR, NMR, and inelastic neutron scattering previously confirmed this high-temperature magnetic behavior. More recently, strain-engineered thin films of La3Ni2O7 showed superconductivity with a critical temperature of about 40K under sufficient compressive strain, while unstrained films reverted to the $(pi/2,pi/2)$ spin stripe order.
Microscopic Hamiltonian and Quasi-One Dimensionality
To understand this magnetic puzzle, researchers deployed state-of-the-art density matrix renormalization group calculations. The team analyzed a microscopic Hamiltonian designed to reflect the crystalline symmetry of La3Ni2O7, incorporating both $d{z^2}$ and $d{x^2-y^2}$ orbitals.
The calculations revealed that the diagonal spin stripes stem from a hidden quasi-one-dimensional behavior within the crystal structure. This order remains stable across a range of electron concentrations when paired with a sizable Hund’s coupling, $J_H$. By setting the hopping parameter $t’$ close to zero in simplified models, the team observed similarities to a one-dimensional zig-zag Kondo-Hubbard lattice, which naturally hosts period-4 charge density waves at quarter filling.
Did you know? Unlike many conventional materials where spin stripe patterns emerge only near absolute zero, La3Ni2O7 maintains its magnetic order up to roughly 150K at ambient pressure.
Pressure, Structural Symmetry, and Interlayer Coupling
In this high-pressure regime, the hopping parameter $t’$ approaches $t$, creating a uniform electronic environment.
The research demonstrates that interlayer antiferromagnetic coupling, $Jperp$, plays a critical role when the material enters this symmetric state. According to the study, a sufficiently large $Jperp$ significantly enhances interlayer pairing tendencies, providing a direct link between the material’s magnetic interactions and its high-temperature superconductivity.
Frequently Asked Questions
What causes the diagonal spin stripe order in La3Ni2O7?
According to researchers from The Chinese University of Hong Kong and Stanford University/SLAC National Accelerator Laboratory, the spin stripe order arises from a hidden quasi-one dimensionality coupled with a sizable Hund’s coupling $J_H$ at ambient pressure.
How does pressure affect La3Ni2O7?
Applying hydrostatic pressure or compressive strain transitions the crystal structure toward a more symmetric arrangement, where interlayer antiferromagnetic coupling $J_perp$ enhances pairing tendencies and induces superconductivity.
What transition temperatures does La3Ni2O7 achieve?
Under high pressure, La3Ni2O7 exhibits superconducting transition temperatures as high as 80K, while strain-engineered thin films display superconductivity around 40K under compressive strain.