Quantifying the electronic reconstruction in LaTiO$_3$/LaNiO$_3$/(LaAlO$_3$)$_3$ heterostructures using RIXS*

A novel approach for manipulating the 3d state in transition metal oxide heterostructures has emerged with the growth of trilayer nickelate LaTiO$_3$/LaNiO$_3$/(LaAlO$_3$)$_3$ (LTNAO) (Disa et al., PRL 114 026801 (2015)). This heterostructure induces a striking reconstruction of the LaNiO$_3$ electronic structure, which is due to a combination of charge transfer from Ti's 3d state and octahedral elongation along the $c$ axis. We use resonant inelastic x-ray scattering (RIXS) experiments at Ni L$_{2,3}$ and O K edges to spectroscopically resolve the LTNAO electronic structure. Surprisingly, our results show that the octahedral elongation generates minor changes in crystal fields at Ni's 3d state compared to bulk LaNiO$_3$. Instead, heterostructuring creates an anisotropic reconstruction of the Ni 3d - O 2p hybridization. The x$^2$-y$^2$ orbital is significantly more hybridized with O p, leading to a 3z$^2$-r$^2$/x$^2$-y$^2$ hole ratio of $\sim$0.55 and large orbital polarization as measured by x-ray absorption spectroscopy. This work establishes RIXS as an ultra-sensitive probe of complex oxide heterostructures.