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

Abstract

"Infinite layer" nickelates (ILNs) ${\cal R}$NiO$_2$ (${\cal R}$=rare earth), having empty apical O sites, become superconducting upon hole doping, stimulating research into the related sequence Nd$_{n+1}$Ni$^{+p}_n$O$_{2n+2}$, formal charge state $p$=1+$\frac{1}{n}$, $n$=2,3,4,...., with the $n$=5 member being found to be superconducting. The two layer system La$_3$Ni$_2$O$_{7-\delta}$, with $\delta$=0,$\frac{1}{2}$,1 ($p$=2.5,2,1.5) approaches the peak in the nickelate superconducting dome but shows no superconductivity. Newly reported La$_3$Ni$_2$O$_5$F reaches the Ni$^{1+}$ goal while, as we show, introducing a partially occupied electron band $E^*$, based on an interstitial density that extends over the three open "apical" layers and leads to a single cylindrical electron Fermi surface giving self-doping. The commonly inert Ni $d_{xz},d_{yz}$ orbitals partner with interstitial $E^*$ to provide an incipient non-analytic Dirac point, with the critical point being reachable by pressure or further F insertion. The $E^*$ electron cylinder and the conventional Ni $dp\sigma$ hole carriers combine to provide a two-fluid dichotomy of hole and electron quasiparticles, affecting normal state properties that should verify the dichotomous aspect of transport.