Electronic and magnetic excitations in La_3Ni_2O_7

The striking discovery of high-temperature superconductivity (HTSC) of 80 K in a bilayer nickelate La_3Ni_2O_7 under a moderately high pressure of about 14 GPa ignited a new wave of studying HTSC in nickelates. The properties of the parental phase at ambient pressure may contain key information on basic interactions therein and bosons that may mediate pairing giving birth to superconductivity. Moreover, the bilayer structure of La_3Ni_2O_7 may suggest a distinct minimal model in comparison to cuprate superconductors. Here using X-ray absorption spectroscopy and resonant inelastic X-ray scattering, we studied La_3Ni_2O_7 at ambient pressure, and found that Ni 3d_x^2-y^2, Ni 3d_z^2, and ligand oxygen 2p orbitals dominate the low-energy physics with a small charge-transfer energy. Remarkably, well-defined optical-like magnetic excitations were found to soften into a quasi-static spin-density-wave ordering, evidencing the strong electronic correlations and rich magnetic properties. Based on a Heisenberg spin model, we found that the inter-layer effective magnetic superexchange interaction is much larger than the intra-layer ones, and proposed two viable magnetic structures. Our results set the foundation for further exploration of La_3Ni_2O_7 superconductor.