Assessing the formation of spin and charge stripes in La₃Ni₂O₇ from first-principles

We employ correlated density-functional theory methods (DFT + Hubbard U) to investigate the spin-density wave state of the bilayer Ruddlesden-Popper (RP) nickelate La₃Ni₂O₇ which becomes superconducting under pressure. We predict that the ground state of this bilayer RP material is a single spin-charge stripe phase with in-plane up^/up/down^/down diagonal stripes with up^/down^ being low spin (formally Ni³⁺: d⁷) and up/down being high spin (formally Ni²⁺: d⁸). The main feature of this solution (that is insulating even at $U=0$) is the dominant role of d_x²-y² bands around the Fermi level, which would become doped with the introduction of electrons via oxygen vacancies. In spite of the similarity with cuprates in terms of the dominant role of d_x²-y² bands, some differences are apparent in the magnetic ground state of La₃Ni₂O₇: the antiferromagnetic out-of-plane coupling within the bilayer (linked to the dz² orbitals forming a spin-singlet-like configuration) is found to be the dominant one while in-plane interactions are reduced due to the stripe order of the ground state. With pressure, this striped magnetic ground state remains similar in nature but the increase in bandwidth quickly transitions La₃Ni₂O₇ into a metallic state with all the activity close to the Fermi level involving, to a large extent, dx²-y² orbitals. This is reminiscent of the cuprates and may provide key insights into how superconductivity arises in this material under pressure.