A combination of density functional and dynamical mean field theory calculations are used to show that the remarkable metal-insulator transition in the rare earth nickelate perovskites arise from a site-selective Mott phase, in which the d-electrons on a half of the Ni ions are localized to form a fluctuating moment while the d-electrons on other Ni ions form a singlet with holes on the surrounding oxygen ions. The calculation reproduces key features observed in the nickelate materials, including an insulating gap in the paramagnetic state, a strong variation of static magnetic moments among Ni sites and an absence of "charge order". A connection between structure and insulating behavior is documented. The site-selective Mott transition may be a more broadly applicable concept in the description of correlated materials.

Since the discovery of the metallic antiferromagnetic (AF) ground state near superconductivity in iron pnictide superconductors 1-3 , a central question has been whether magnetism in these materials arises from weakly correlated electrons 4,5 , as in the case of spin density wave in pure chromium 6 , requires strong electron correlations 7 , or can even be described in terms of localized electrons 8,9 such as the AF insulating state of copper oxides 10 . Here we use inelastic neutron scattering to determine the absolute intensity of the magnetic excitations throughout the Brillouin zone in electron-doped superconducting BaFe 1.9 Ni 0.1 As 2 (T c = 20 K), which allows us to obtain the size of the fluctuating magnetic moment m 2 , and its energy distribution 11,12 . We find that superconducting BaFe 1.9 Ni 0.1 As 2 and AF BaFe 2 As 2 (ref. 13) both have fluctuating magnetic moments m 2 ≈ 3.2 µ 2 B per Fe(Ni), which are similar to those found in the AF insulating copper oxides 14,15 . The common theme in both classes of high-temperature superconductors is that magnetic excitations have partly localized character, thus showing the importance of strong correlations for high-temperature superconductivity 16 .In the undoped state, iron pnictides such as BaFe 2 As 2 form a metallic low-temperature orthorhombic phase with the antiferromagnetic (AF) structure as shown in Fig. 1a (ref. 17). Inelastic neutron scattering measurements have mapped out spin waves throughout the Brillouin zone in the AF orthorhombic and paramagnetic tetragonal phases 13 . On Co-and Ni-doping to induce optimal superconductivity via electron doping, the orthorhombic structural distortion and static AF order in BaFe 2 As 2 are suppressed and the system becomes tetragonal and paramagnetic at all temperatures 18 . In previous inelastic neutron scattering experiments on optimally electron-doped Ba(Fe, Co, Ni) 2 As 2 superconductors 11,12,[19][20][21][22] , spin excitations up to ∼120 meV were observed. However, the lack of spin excitation data at higher energies in absolute units precluded a comparison with spin waves in undoped BaFe 2 As 2 . Only the absolute intensity measurements in the entire Brillouin zone can reveal the effect of electron doping on the overall spin excitation spectra and allow a direct comparison with the results in the AF insulating copper oxides 14,15 . For the experiments, we chose to study well-characterized electron-doped BaFe 1.9 Ni 0.1 As 2 (refs 20,22) because large single crystals were available 23 and their properties are similar to Co-doped BaFe 2 As 2 (refs 11,12,19,21,24).By comparing spin excitations in BaFe 1.9 Ni 0.1 As 2 and BaFe 2 As 2 throughout the Brillouin zone, we were able to probe how electron doping and superconductivity affect the overall spin

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