Figure 2.

Mott insulator, self-doping effect, and possible Kondo coupling in nickelates RNiO₂

(A) Sketch of a Mott insulator and a charge transfer insulator. The blue and pink domes stand for the lower and upper Hubbard bands, while the orange one shows the O 2p band, assuming no p-d hybridization.

(B) XES and XAS in the pre-edge region of LaNiO₂, roughly reflecting the occupied and unoccupied oxygen PDOS, respectively.

(C) RIXS intensity map of LaNiO₂ and NdNiO₂. The corresponding XASs are superimposed as a solid black line in each map. The dashed boxes highlight the 0.6 eV features of LaNiO₂ and NdNiO₂ that are associated with the Ni–La and Ni–Nd hybridizations, respectively. Adapted from Hepting et al.⁷⁸

(D) Illustration of the effective model on a two-dimensional square lattice of NiO₂ plane in RNiO₂. The blue ball with an arrow represents the local spin 1/2 on Ni site, which interacts with its neighboring spin by AF coupling J. The yellow ball with an arrow denotes a Nd 5d electron, which couples to Ni spin by the Kondo coupling K, to form a Kondo singlet. Red open circle represents Ni 3d⁸ configuration, or a holon. The t and t∗ are the hopping integrals of Kondo singlet and holon, respectively. Adapted from Zhang et al.⁸¹