Origin of Spin Stripes in Bilayer Nickelate La₃Ni₂O₇

Summary

This paper proposes a microscopic Hamiltonian that faithfully reflects the crystal symmetry of the bilayer nickelate La₃Ni₂O₇ under ambient pressure, addressing its unconventional magnetic order. Large-scale density matrix renormalization group calculations reveal that under a large Hund coupling (J_H), a ((pi/2, pi/2)) spin stripe order emerges due to hidden quasi-one-dimensionality and persists over a range of electron concentrations. In the more symmetric high-pressure regime, when the interlayer antiferromagnetic coupling (J_perp) is sufficiently strong, the model exhibits an enhanced tendency for interlayer pairing. This study unveils the microscopic origin of the diagonal spin stripe order and identifies both the Hund coupling (J_H) and the interlayer coupling (J_perp) as key factors controlling the magnetic order and pairing tendency in La₃Ni₂O₇.

Materials

• La₃Ni₂O₇

Methods

• density matrix renormalization group (DMRG)
• microscopic Hamiltonian

Keywords

• spin stripe order
• hund's coupling
• interlayer antiferromagnetic coupling

Highlights

• Hidden quasi-one dimensionality of spin stripe order.
• Indispensable role of Hund's coupling.

Conclusions

• Spin stripe order (π/2,π/2) emerges in La₃Ni₂O₇ with sizable Hund's coupling, and superconducting tendency appears when interlayer antiferromagnetic coupling becomes large under pressure.

Main claims

• A microscopic model with non-uniform hopping and Hund's coupling reproduces the (π/2,π/2) spin stripe order observed in La₃Ni₂O₇.
  • Evidence: Full text: DMRG calculations show (π/2,π/2) spin stripe order in the model for t' < t and large U.,Abstract: 'Using state-of-the-art density matrix renormalization group calculations, we show that (π/2,π/2) spin stripe order emerges in our model'
• Superconductivity in La₃Ni₂O₇ requires large interlayer antiferromagnetic coupling, which naturally occurs under pressure.
  • Evidence: Abstract: 'Our model exhibits superconducting tendency only when the interlayer antiferromagnetic coupling J⊥ becomes sufficiently large, which naturally occurs under pressure.',Full text: DMRG results show power-law decay of interlayer singlet pairing correlation with exponent Ksc < 2 for J⊥=4t.
• Hund's coupling JH plays an indispensable role in stabilizing the spin stripe order.
  • Evidence: Abstract: 'The spin stripe order occurs over a range of electron concentrations, but requires a sizable Hund's coupling JH.',Full text: 'We have confirmed numerically that by taking a smaller JH, one needs a larger U to stabilize the stripe order.'

Workflow

• Model Construction — The model describes the interplay of lattice structure and magnetism in La₃Ni₂O₇.
  • Materials: Microscopic Hamiltonian for La₃Ni₂O₇; dx2-y2 and dz2 orbitals; Hund's coupling JH; interlayer coupling J⊥
  • Methods: Construction of effective bilayer two-orbital model
  • Observations: Model captures strain effects via hopping anisotropy
• DMRG Calculation — DMRG results show (π/2,π/2) spin stripe order emerges when t' < t and U is sufficiently large, and superconductivity appears when J⊥ is large.
  • Materials: GPU-accelerated DMRG; System sizes up to 400 sites; Bond dimension up to 20000
  • Methods: Density matrix renormalization group (DMRG) calculations; Subspace expansion technique
  • Observations: Spin correlation functions F(|ri-rj|); Phase diagrams in t'-U plane for fixed JH
• Phase Diagram Analysis — The spin stripe order requires sizable Hund's coupling, and superconductivity occurs when interlayer coupling is sufficiently large.
  • Materials: Phase diagrams from DMRG; Spin correlation functions
  • Methods: Identification of spin stripe orders (π/2,π/2) and (π,0); Analysis of superconducting correlations (interlayer singlet pairing)
  • Observations: (π/2,π/2) SSO stable when decoupled 1D chains are ferromagnetic; Superconducting power-law decay r-Ksc with Ksc < 2 for large J⊥