Origin of the Diagonal Double-Stripe Spin-Density-Wave and Potential Superconductivity in Bulk La3Ni2O7 at Ambient Pressure

The discovery of high-temperature superconductivity (SC) with T _c ≈ 80 K in the pressurized La ₃ Ni ₂ O ₇ has aroused great interests. Currently, due to technical difficulties, most experiments on La ₃ Ni ₂ O ₇ can only be performed at ambient pressure (AP). Particularly, various experiments have revealed the presence of spin density wave (SDW) in the unidirectional diagonal double-stripe pattern with wave vector near (π/2, π/2) in La ₃ Ni ₂ O ₇ at AP. In this work, we employ first-principle calculations followed by the random phase approximation (RPA)-based study to clarify the origin of this special SDW pattern and the potential SC in La ₃ Ni ₂ O ₇ at AP. Starting from our density-functional-theory band structure, we construct an eight-band bilayer tight-binding model using the Ni-3 d _{z ²} and 3 d _{x ² −y ²} orbitals, which is equipped with the standard multi-orbital Hubbard interaction. Our RPA calculation reveals an SDW order driven by Fermi-surface nesting with wave vector Q ≈ (0, 0.84π) in the folded Brillouin zone (BZ). From the view of the unfolded BZ, the wave vector turns to Q ₀ ≈ (0.42π, 0.42π), which is near the one detected by various experiments. Further more, this SDW exhibits an interlayer antiferromagnetic order with a unidirectional diagonal double-stripe pattern, consistent with recent soft X-ray scattering experiment. This result suggests that the origin of the SDW order in La ₃ Ni ₂ O ₇ at AP can be well understood in the itinerant picture as driven by Fermi surfaces nesting. In the aspect of SC, our RPA study yields an approximate s ^± -wave spin-singlet pairing with T _c much lower than that under high pressure. Further more, the T _c can be strongly enhanced through hole doping, leading to possible high-temperature SC at AP.