Parameter transferability, self-doping, and metallicity in ${\mathrm{LaNiO}}_3/{\mathrm{LaMnO}}_3$ superlattices

Motivated by recent experiments, we use the $+U$ extension of the generalized gradient approximation to density functional theory to study superlattices composed of alternating layers of ${\mathrm{LaNiO}}_3$ and ${\mathrm{LaMnO}}_3$. For comparison we also study a rocksalt [(111) double perovskite] structure and bulk ${\mathrm{LaNiO}}_3$ and ${\mathrm{LaMnO}}_3$. A Wannier function analysis indicates that band parameters are transferable from bulk to superlattice situations with the exception of the transition-metal $d$-level energy, which has a contribution from the change in $d$-shell occupancy. The charge transfer from Mn to Ni is found to be moderate in the superlattice, indicating metallic behavior, in contrast to the insulating behavior found in recent experiments, while the rocksalt structure is found to be insulating with a large Mn-Ni charge transfer. We suggest a high density of cation antisite defects may account for the insulating behavior experimentally observed in short-period superlattices.

Figure 1

Model representation of the structures studied in this paper. (a) Cubic ${\mathrm{LaNiO}}_3$ or ${\mathrm{LaMnO}}_3$, (b) multilayer of ${\left[{\mathrm{LaMnO}}_3\right]}_1/{\left[{\mathrm{LaNiO}}_3\right]}_1$, and (c) rocksalt structure of ${\mathrm{LaMnO}}_3$ and ${\mathrm{LaNiO}}_3$. In (c) the octahedra represent the sixfold coordination of the metallic cations.

Figure 2

Minority- (left subpanels) and majority-spin (right subpanels) bands of cubic ${\mathrm{LaMnO}}_3$, ${\mathrm{LaNiO}}_3$, the (001) ${\left({\mathrm{LaNiO}}_3\right)}_1/{\left({\mathrm{LaMnO}}_3\right)}_1$ superlattice, and the rocksalt (111) ${\mathrm{La}}_2{\mathrm{MnNiO}}_6$ double perovskite. All calculations are performed for ferromagnetic ground states using the GGA+$U$ method as described in the text. The bands are plotted along the high-symmetry lines $\mathrm{\Gamma }\to X\to R\to \mathrm{\Gamma }\to M\to X$ of the Brillouin zone.

Figure 3

Projection of the majority-spin density of states of ferromagnetic cubic ${\mathrm{LaMnO}}_3$ and ${\mathrm{LaNiO}}_3$ onto the transition-metal (Mn or Ni) $d$ orbitals and onto oxygen $p$ orbitals. The $e_g$-symmetry and $t_{2g}$-symmetry orbitals are plotted separately, as are the oxygen $p_{\sigma }$ (hybridizing with the transition-metal $e_g$ orbitals) and $p_{\pi }$ (not hybridizing with the transition-metal $e_g$ orbitals).

Figure 4

Projection of the majority-spin density of states of the ferromagnetic ${\left({\mathrm{LaNiO}}_3\right)}_1/{\left({\mathrm{LaMnO}}_3\right)}_1$ superlattice onto the transition-metal (Mn or Ni) $d$ orbitals and onto oxygen $p$ orbitals. The two $e_g$ orbitals are plotted separately, but all three $t_{2g}$ orbitals are added together. The oxygen $p_{\sigma }$ (hybridizing with the transition-metal $e_g$ orbitals) and $p_{\pi }$ (not hybridizing with the transition-metal $e_g$ orbitals) are distinguished.

Figure 5

Projections of the majority-spin density of states of ferromagnetic rocksalt $A_2{BB}^{\prime }{\mathrm{O}}_6$ onto $e_g$ and $t_{2g}$ orbitals are plotted separately. The oxygen $p_{\sigma }$ (hybridizing with the transition-metal $e_g$ orbitals) and $p_{\pi }$ (not hybridizing with the transition-metal $e_g$ orbitals) are distinguished.

Figure 6

(a) Schematic representation of the relaxed cubic ${\left({\mathrm{LaNiO}}_3\right)}_2/{\left({\mathrm{LaMnO}}_3\right)}_2$ heterostructure. (b) Band structure of the majority spin. The projected DOS onto the $d$ orbitals of the two types of nonequivalent Mn atoms is plotted in (c) and (d). The bands are plotted along the high-symmetry lines $\mathrm{\Gamma }\to X\to R\to \mathrm{\Gamma }\to M\to X$ of the Brillouin zone.

Figure 7

Comparison of the first-principles-obtained band structure with the wannier90-interpolated band structure for cubic ${\mathrm{LaMnO}}_3$, cubic ${\mathrm{LaNiO}}_3$, the ${\left({\mathrm{LaNiO}}_3\right)}_1/{\left({\mathrm{LaMnO}}_3\right)}_1$ heterostructure, and ferromagnetic rocksalt.

Figure 8

Minority- (left subpanels) and majority-spin (right subpanels) bands of the $2\times 2\times 4{\left({\mathrm{LaNiO}}_3\right)}_2/{\left({\mathrm{LaMnO}}_3\right)}_2$ heterostructure for different sets of $+U$ values ($J=1$). (a) $U_{Mn}=6$, $U_{Ni}=8$ and (b) $U_{Mn}=2$, $U_{Ni}=4$. The bands are plotted along the high-symmetry lines $\mathrm{\Gamma }\to X\to R\to \mathrm{\Gamma }\to M\to X$ of the Brillouin zone.