Interplay between electron correlations, magnetic state, and structural confinement in LaNiO$_3$ ultrathin films

We report a theoretical study of the effects of electron correlations and structural confinement on the electronic properties and magnetic state of LaNiO$_3$ (LNO) thin films epitaxially deposited on the $(001)$ LaNiO$_3$ (LAO) substrate. Using the DFT + U method we compute the electronic band structure, magnetic properties, and phase stability of the 1.5 unit-cell-thick NiO$_2$-terminated LNO thin films. Our results reveal complex diversity of the electronic states caused by the effects of structural confinement, interfacial charge transfer and electronic correlations. Our calculations suggest the appearance of in-plane (110) charge disproportionation of the Ni ions in the interface NiO$_2$ layer of the antiferromagnetically ordered LNO thin films. Moreover, the electronic states of both the antiferromagnetic and ferromagnetic LNO/LAO show a large orbital polarization of the Ni ions in the surface NiO$_2$ layers. Our results suggest the crucial importance of oxygen defects to explain the metal-to-insulator phase transition experimentally observed in a few-unit-cell-thick LNO/LAO thin films.