Electronic structure and orbital polarization of LaNiO${}_3$ with a reduced coordination and under strain: A first-principles study

First-principles density functional theory calculations have been performed to understand the electronic structure and orbital polarization of LaNiO${}_3$ with a reduced coordination and under strain. From the slab calculation to simulate the [001] surface, it is found that $d_{3z^2-r^2}$ orbital occupation is significantly enhanced relative to $d_{x^2-y^2}$ occupation, owing to the reduced coordination along the perpendicular direction to the sample plane. Furthermore, the sign of the orbital polarization does not change under external strain. The results are discussed in comparison to the bulk and heterostructure cases, which sheds new light on the understanding of the available experimental data.

Figure 1

Unit-cell structure of the double-layer slab calculation. Large (red), middle (gray), and small (blue) spheres represent La, Ni, and O atoms, respectively. Ni(1) and Ni(2) correspond to the surface-like and bulk-like Ni sites, respectively.

Figure 2

(a) Projected DOS and (b) integrated DOS of Ni $e_g$ states in a (LNO)${}_1$/(LAO)${}_1$ heterostructure. (c) Projected DOS and (d) integrated DOS of Ni $e_g$ states in a single-layer LNO slab. Red (solid) and blue (dashed) lines correspond to $d_{3z^2-r^2}$ and $d_{x^2-y^2}$ states, respectively. The Fermi level is set to be 0.

Figure 3

Projected DOS for Ni $e_g$ states in an LNO/LAO heterostructure (a, b) and a single-layer slab (c, d). (a, c) Calculated under compressive strain ($-3\%$ in-plane lattice mismatch); (b, d) calculated under tensile strain (+3$\%$ in-plane lattice mismatch). Red (solid) and blue (dashed) lines correspond to $d_{3z^2-r^2}$ and $d_{x^2-y^2}$ states, respectively. The Fermi level is set to be 0.

Figure 4

Calculated orbital polarizations for an LNO/LAO heterostructure (blue), bulk LNO (green), and unrelaxed (red) and relaxed (orange) slabs as a function of strain. The horizontal dotted line shows the zero polarization points. The relaxed result for heterostructure (magenta) is from Ref. 19 with renormalization.

Figure 5

Projected Ni-$e_g$ DOS in the double-layer slab structure. (a) The top-layer Ni site, which has only five neighboring oxygens; (b) the second-layer Ni site, which has six neighbors. Red (solid) and blue (dashed) lines correspond to $d_{3z^2-r^2}$ and $d_{x^2-y^2}$ states, respectively, and the Fermi level is set to be 0.