Exchange Interactions And Sensitivity Of The Ni Two-Hole Spin State To Hund'S Coupling In Doped Ndnio2

Using the density-functional-based LDA+U method and linear-response theory, we study the magnetic exchange interactions of the superconductor Nd1-xSrxNiO2. Our calculated nearest-neighbor exchange constant J(1) = 82 meV is large, weakly affected by doping, and is only slightly smaller than that found in the sister compound CaCuO2. However, we find that the hole doping significantly enhances the interlayer exchange coupling as it affects the magnetic moment of the Ni-3d(3z2-r2) orbital. This can be understood in terms of the small hybridization of Ni-3d(3z2-r2) within the NiO2 plane, which results in a flat band near the Fermi level, and its large overlap along the z direction. We also demonstrate that the Nd-5d states appearing at the Fermi level do not affect the magnetic exchange interactions, and thus they may not participate in the superconductivity of this compound. Whereas many previous works emphasized the importance of the Ni-3d(x2-y2) and Nd-5d orbitals, we analyze instead the solution of the Ni-3d(x2y2)/Ni-3d(3z2-r2) minimal model using dynamical mean field theory. It reveals an underlying Mott insulating state that, depending on the precise values of the intra-atomic Hund's coupling smaller or larger than 0.83 eV, selects upon doping either S = 0 or 1 two-hole states at low energies, leading to very different quasiparticle band structures. We propose that trends upon doping in the spin excitation spectrum and the quasiparticle density of states can be a way to probe the Ni 3d(8) configuration.