Fig. 2.

Electronic evolution from perovskite ${\mathrm{N}\mathrm{d}\mathrm{N}\mathrm{i}\mathrm{O}}_3$ (black) to infinite-layer ${\mathrm{N}\mathrm{d}\mathrm{N}\mathrm{i}\mathrm{O}}_2$ (blue) phase for epitaxial thin films on ${\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_3$. (A) The Ni-${\mathrm{L}}_{\mathrm{2,3}}$ edge (3$d$ states) shows a clear change upon O reduction from ${\mathrm{N}\mathrm{d}\mathrm{N}\mathrm{i}\mathrm{O}}_3$ to ${\mathrm{N}\mathrm{d}\mathrm{N}\mathrm{i}\mathrm{O}}_2$, including significant broadening and shift of the ${\mathrm{L}}_3$ edge (B). (C) The Nd-${\mathrm{M}}_{\mathrm{4,5}}$ edge (4$f$ states) shows little or no change between the two compounds. Spectra in A and C are normalized by integrated signal over the full energy ranges shown; the Ni-${\mathrm{L}}_3$ edges in B have been renormalized by maximum intensity for easier comparison.