Graded orbital occupation near interfaces in a La₂NiO ₄-La₂CuO₄ superlattice

X-ray absorption spectroscopy and resonant soft x-ray reflectivity show a nonuniform distribution of oxygen holes in a La₂NiO ₄-La₂CuO₄ (LNO-LCO) superlattice, with excess holes concentrated in the LNO layers. Weak ferromagnetism with T_c=160 K suggests a coordinated tilting of NiO₆ octahedra, similar to that of bulk LNO. Ni d_3z2-r2 orbitals within the LNO layers have a spatially variable occupation. This variation of the Ni valence near LNO-LCO interfaces is observed with resonant soft x-ray reflectivity at the Ni and Cu L edges, at a reflection suppressed by the symmetry of the structure, and is possible through graded doping with holes, due to oxygen interstitials taken up preferentially by inner LNO layers. Since the density of oxygen atoms in the structure can be smoothly varied with standard procedures, this orbital occupation, robust up to at least 280 K, is tunable.