Effects of the apical-oxygen vacancy and magnetism on the ultra-thin LaNiO3 layer studied using DFT+DMFT

While the bulk LaNiO3 remains metallic at all temperatures, the ultrathin films of LaNiO3 shows a metal-insulator transition as the layer thickness reduces to 2 unit cells. Recent experiment shows that the NiO2 surface layer without top LaO layer can have higher resistivity than the same NiO2 layer with apical oxygens. In this talk, I will present the density functional theory plus dynamical mean field theory (DFT+DMFT) calculations for the one-layer LaNiO3 slab including the LaAlO3 substrate and the vacuum. The slab geometry with the capping LaO layer is metallic even with DFT+DMFT although Ni eg orbitals are renormalized compared to DFT bands. We also move the top LaO layer away from the surface layer to simulate the effect of apical oxygens on the electronic structure. As the apical oxygen moves away from the Ni ion, an effective charge transfer occurs toward the dz2 orbital and the Wannier orbital level is shifted below the dx2-y2 orbital. Moreover, the DMFT self energy makes the dz2 orbital more incoherent and it eventually becomes Mott insulating in the absence of apical oxygens. We also simulate the antiferromagnetic calculations with the up-up-down-down spin structure as measured in experiment. The results will be also compared with the above paramagnetic calculations.