Role of 4f states in infinite-layer NdNiO2

Atomic $4f$ states have been found to be essential players in the physical behavior of lanthanide compounds, at the Fermi level ${E}_{F}$ as in the proposed topological Kondo insulator ${\mathrm{SmB}}_{6}$, or further away as in the magnetic superconductor system $R{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ ($R$ = rare-earth ion) and in ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$, where the $4f$ shell of Pr has a devastating effect on superconductivity. In hole-doped $R{\mathrm{NiO}}_{2}$, the $R$ = Nd member is found to be superconducting while $R$ = La is not, in spite of the calculated electronic structures being nearly identical. We report first-principles results that indicate that the Nd $4f$ moment affects states at ${E}_{F}$ in infinite-layer ${\mathrm{NdNiO}}_{2}$, an effect that will not occur for ${\mathrm{LaNiO}}_{2}$. Treating 20% hole doping in the virtual crystal approach indicates that 0.15 holes empty the $\mathrm{\ensuremath{\Gamma}}$-centered Nd-derived electron pocket while leaving the other electron pocket unchanged; hence Ni only absorbs 0.05 holes; the La counterpart would behave similarly. However, coupling of $4f$ states to the electron pockets at ${E}_{F}$ arises through the Nd intra-atomic $4f\text{\ensuremath{-}}5d$ exchange coupling $K\ensuremath{\approx}0.5\mathrm{eV}$ and is ferromagnetic (FM), i.e., anti-Kondo, in sign. This interaction causes spin-disorder broadening of the electron pockets and should be included in models of the normal and superconducting states of ${\mathrm{Nd}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_{2}$. The Ni moments differ by $0.2{\ensuremath{\mu}}_{B}$ for FM and antiferromagnetic alignment (the latter are larger), reflecting some itineracy and indicating that Heisenberg coupling of the moments may not provide a quantitative modeling of Ni-Ni exchange coupling.