Daily Overview: Today’s highlight focuses on an in-depth understanding of the electronic structure of hybrid Ruddlesden–Popper nickelates. Through first-principles calculations, Song et al. identified a bipartite electronic structure in the bilayer nickelate La₃Ni₂O₅F, in which a Ni dpσ hole-type Fermi surface coexists with the cylindrical Fermi surface of an interstitial electron-type E* band. In this structure, self-doping by the interstitial states gives an effective Ni valence of about +1.09 and creates nearly nonanalytic Dirac points near the M point, which can be tuned by pressure or further fluorine insertion. The work further reveals that the interstitial density induces an even–odd splitting of approximately 1 eV in the Ni d_{z²} orbital. This two-fluid behavior, characterized by coexisting hole-type dpσ quasiparticles and interstitial electron-type quasiparticles, will significantly affect normal-state transport and far-infrared properties and may promote unconventional superconductivity. arXiv submission processing window: 2026-08-28 00:00 to 2026-08-28 00:00 UTC.

1. Dichotomous electronic system in a bilayer Ni$^{1+}$ nickelate

Summary: Using first-principles methods, this work studies the electronic structure of the bilayer nickelate La₃Ni₂O₅F, in which the disordered O/F layers are treated by the virtual crystal approximation. It is found that the system possesses an ideal two-dimensional electronic structure, with the Ni dpσ band forming a hole-type Fermi surface at the M point; meanwhile, an E* band originating from interstitial density and not belonging to any atomic orbital is partially occupied, forming a cylindrical electron Fermi surface along M–A and producing a self-doping of about 0.18 electrons, resulting in an effective Ni valence of about +1.09. In addition, the normally inert Ni d_{xz}/d_{yz} orbitals and the E* band form a nearly non-analytic Dirac point near the M point, and this critical point can be realized by applying pressure or by further fluorine insertion; the interstitial density also induces a parity splitting of about 1 eV in the Ni d_{z²} orbital. These results indicate that La₃Ni₂O₅F hosts a dichotomous two-fluid behavior in which hole-type dpσ quasiparticles and interstitial electron-type quasiparticles coexist, which will significantly affect the normal-state transport and far-infrared properties and may promote an unconventional superconducting state.