Role of 4f states in infinite-layer NdNiO2

Abstract

Atomic 4f states have been found to be essential players in the physical behavior of lanthanide compounds, at the Fermi level EF as in the proposed topological Kondo insulator SmB6, or further away as in the magnetic superconductor system RNi2B2C (R = rare-earth ion) and in Y1-xPrxBa2Cu3O7, where the 4f shell of Pr has a devastating effect on superconductivity. In hole-doped RNiO2, 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 EF in infinite-layer NdNiO2, an effect that will not occur for LaNiO2. Treating 20% hole doping in the virtual crystal approach indicates that 0.15 holes empty the Γ-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 EF arises through the Nd intra-atomic 4f-5d exchange coupling K≈0.5eV 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 Nd0.8Sr0.2NiO2. The Ni moments differ by 0.2μB 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.