Anomalous Behavior of the Ni${}^{1+}$ moment in bi-infinite-layered La${}_3$Ni${}_2$O${}_5$F

Abstract

The discovery of superconductivity in hole-doped Ni${}^{1+}$ systems with ``infinite layer" NiO${}_2$ square-lattices analogous to the Cu${}^{2+}$ CaCuO${}_2$ cuprate has renewed conflicting pictures of the Cu${}^{2+}$$-$Ni${}^{1+}$ similarities and distinctions. Recent synthesis of formal Ni${}^{1+}$ La${}_3$Ni${}_2$O${}_5$F with two infinite NiO${}_2$ layers per cell provides a novel member of this class. We find that Ni${}^{1+}$ in this material is unusually flexible. First principles density functional theory studies reveal a single partially occupied electron band derived from density in {\it three interstitial} layers that provides self-doping to a Ni${}^{1.09+}$ charge state. The blocking La(O/F)La layer provides isolation of the NiO${}_2$ bilayer to strictly two-dimensional electronic and magnetic systems. Fixed spin moment calculations of magnetic tendencies reveal behavior unlike previous nickelates, including an essentially vanishing differential susceptibility up to a large magnetic field and a metastable ferromagnetic (FM) state. Antiferromagnetic (AFM) order is favored energetically as expected; surprisingly both FM and AFM states are obtained without correlation (Hubbard $U$) corrections. Doping away from half-filling by an interstitial density derived band and likely two-dimensional fluctuations account for the lack of observation of any magnetic transition.

12 pages, 4 embedded figures