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 ${\mathrm{Y}}_{1-x}{\mathrm{Pr}}_x{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$, 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 $\mathrm{\Gamma }$-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 $4f\text{−}5d$ exchange coupling $K\approx 0.5\mathrm{eV}$ 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 $0.2{\mu }_B$ 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.

• Received 7 November 2019

DOI:https://doi.org/10.1103/PhysRevB.101.020503