  The search for oxide materials with physical properties similar to the
cuprate high Tc superconductors, but based on alternative transition metals
such as nickel, has grown and evolved over time. The recent discovery of
superconductivity in doped infinite-layer nickelates RNiO2 (R = rare-earth
element) further strengthens these efforts.With a crystal structure similar to
the infinite-layer cuprates - transition metal oxide layers separated by a
rare-earth spacer layer - formal valence counting suggests that these materials
have monovalent Ni1+ cations with the same 3d electron count as Cu2+ in the
cuprates. Here, we use x-ray spectroscopy in concert with density functional
theory to show that the electronic structure of RNiO2 (R = La, Nd), while
similar to the cuprates, includes significant distinctions. Unlike cuprates
with insulating spacer layers between the CuO2 planes, the rare-earth spacer
layer in the infinite-layer nickelate supports a weakly-interacting
three-dimensional 5d metallic state. This three-dimensional metallic state
hybridizes with a quasi-two-dimensional, strongly correlated state with 3dx2-y2
symmetry in the NiO2 layers. Thus, the infinite-layer nickelate can be regarded
as a sibling of the rare earth intermetallics, well-known for heavy Fermion
behavior, where the NiO2 correlated layers play an analogous role to the 4f
states in rare-earth heavy Fermion compounds. This unique Kondo- or
Anderson-lattice-like "oxide-intermetallic" replaces the Mott insulator as the
reference state from which superconductivity emerges upon doping.
