Abstract
Building upon the recent progress on the intriguing underlying physics for the newly discovered infinite-layer nickelates, in this article we review an examination of valence charge and spin excitations via multi-orbital Hubbard models as way to determine the fundamental building blocks for Hamiltonians that can describe the low energy properties of infinite-layer nickelates. We summarize key results from density-functional approaches, and apply them to the study of x-ray absorption to determine the valence ground states of infinite-layer nickelates in their parent form, and show that a fundamental d9 configuration as in the cuprates is incompatible with a self-doped ground state having holes in both dx2−y2 and a rare-earth-derived axial orbital. When doped, we determine that the rare-earth-derived orbitals empty and additional holes form low spin (S = 0) d8 Ni states, which can be well-described as a doped single-band Hubbard model. Using exact diagonalization for a 2-orbital model involving Ni and rare-earth orbitals, we find clear magnons at 1/2 filling that persist when doped, albeit with larger damping, and with a dependence on the precise orbital energy separation between the Ni- and rare-earth-derived orbitals. Taken together, a full two-band model for infinite-layer nickelates can well describe the valence charge and spin excitations observed experimentally.
Highlights
The discovery of experimental superconductivity in infinite-layer nickelates [1], 20 years after they were theoretically proposed [2], has unveiled new territory to probe the unknowns of unconventional superconductivity
Hybridization with ligands is not considered in this single-site calculation, it can still capture the difference in x-ray absorption (XAS) between high-spin and low-spin ground state as described below
Similar to previous reports by Ref. [17], with light polarized along the z-direction, the intensity of the XAS peaks decrease in the low spin state, while the intensity remains the same order of magnitude for the high spin state
Summary
The discovery of experimental superconductivity in infinite-layer nickelates [1], 20 years after they were theoretically proposed [2], has unveiled new territory to probe the unknowns of unconventional superconductivity. The comparison between the superconducting nickelates to the cuprates has proven to be a rich area of inquiry. Significant progress has been made in the last 2 years, but many mysteries about this novel superconducting family remain unsolved. As pointed out early on [3], the bandstructure for LaNiO2 while being primarily 3d9 Ni near the Fermi level has additional small Fermi pockets of largely axial character involving Ni 3dz and La 5dz orbitals. Including a Hubbard U on nickel splits the d9 states but still leaves itinerant states at the Fermi level, indicating that the parent compound for infinite-layer nickelate is not analogous to undoped CuO2 as an antiferromagnetic Mott insulator. The location of the centroid of the oxygen states in Valence Charge of Infinite-Layer Nickelates
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