Engineering Correlation Effects via Artificially Designed Oxide Superlattices

Ab initio calculations are used to predict that a superlattice composed of layers of ${\mathrm{LaTiO}}_3$ and ${\mathrm{LaNiO}}_3$ alternating along the [001] direction is a $S=1$ Mott insulator with large magnetic moments on the Ni sites, negligible moments on the Ti sites and a charge transfer gap set by the energy difference between Ni $d$ and Ti $d$ states, distinct from conventional Mott insulators. Correlation effects are enhanced on the Ni sites via filling the oxygen $p$ states and reducing the Ni-O-Ni bond angle. Small hole (electron) doping of the superlattice leads to a two-dimensional single-band situation with holes (electrons) residing on the Ni $d_{x^2-y^2}$ (Ti $d_{xy}$) orbital and coupled to antiferromagnetically correlated spins in the ${\mathrm{NiO}}_2$ layer.

Figure 1

(a) Schematic band structures of component materials ${\mathrm{LaTiO}}_3$ and ${\mathrm{LaNiO}}_3$. The dashed purple lines are the Fermi levels for the two materials. ${\mathrm{LaTiO}}_3$ shows insulating behavior with a small excitation gap set by Ti $d\mathrm{\text{−}}d$ transitions and a wide energy separation between Ti $d$ states and O $p$ states. ${\mathrm{LaNiO}}_3$ exhibits metallic behavior with strong mixing between Ni $d$ states and O $p$ states. The red arrow highlights the direction of charge transfer in the superlattice. (b) Schematic band structure of the $({\mathrm{LaTiO}}_3{)}_1/({\mathrm{LaNiO}}_3{)}_1$ superlattice. Ti $d$ states are above the Fermi level (dashed purple line). Correlation effects split Ni $d$ states into lower and upper Hubbard bands, separated by $U_{\mathrm{Ni}}$. (c) Densities of states for majority (above axis) and minority (below axis) spins of superlattice (upper left) and reference materials NiO (lower left), ${\mathrm{LaTiO}}_3$ (upper right; zero of energy is shifted so that oxygen bands align with those of ${\mathrm{LaNiO}}_3$) and ${\mathrm{LaNiO}}_3$ (lower right). The densities of states are obtained using $\mathrm{DFT}+U$ calculations with $U_{\mathrm{Ni}}=6\mathrm{eV}$ and $U_{\mathrm{Ti}}=4\mathrm{eV}$.

Figure 2

Side view (a) and top view (b) of the unit cell of the $({\mathrm{LaTiO}}_3{)}_1/({\mathrm{LaNiO}}_3{)}_1$ superlattice simulated here. The atom positions are obtained from a representative relaxed structure found in our first-principles calculations. La atoms are green, Ti atoms are blue, Ni atoms are black, and O atoms are red. The oxygen octahedra, shaded orange, enclose Ni atoms and the oxygen octahedra, shaded dark blue, enclose Ti atoms. The side view (a) shows a weak tilting of oxygen octahedra. The top view (b) highlights a strong rotation of oxygen octahedra.

Figure 3

Panels (a)–(c): Electronic and magnetic properties of the $({\mathrm{LaTiO}}_3{)}_1/({\mathrm{LaNiO}}_3{)}_1$ superlattice for different magnetic orderings ($F$ denotes ferromagnetic ordering, $S$ denotes $S$-type or stripe antiferromagnetic ordering, and $G$ denotes $G$-type or checkerboard-patterned antiferromagnetic ordering) as a function of $U_{\mathrm{Ni}}$ with $U_{\mathrm{Ti}}=4\mathrm{eV}$. (a) Energy difference per Ni between different magnetic orderings. (b) Minimum excitation gap. (c) Absolute value of site projected magnetization of Ni $d$ states. (d) Metal-insulator boundaries for each magnetic ordering on the ($U_{\mathrm{Ni}}$, $U_{\mathrm{Ti}}$) phase diagram, respectively. $F$, $S$, and $G$ have the same meaning as in (a)–(c). “$M$” and “$I$” denote metallic and insulating states. The open symbols denote the upper limits of $U_{\mathrm{Ni}}$ for which metallic phases are found, while the solid symbols indicate the lower limits of $U_{\mathrm{Ni}}$ for which insulating phases have been found (note that only integer values of $U$ have been studied).

Figure 4

Comparison of densities of states for ferromagnetic ($F$), stripe antiferromagnetic ($S$), and checkerboard ($G$) magnetic orderings calculated at $U_{\mathrm{Ni}}=6\mathrm{eV}$ and $U_{\mathrm{Ti}}=4\mathrm{eV}$. The conduction band minimum is aligned (highlighted by the orange line). The energy gap between the conduction band minimum and Ni $d$ main peak (denoted by the maroon arrow) is approximately the same for different magnetic orderings.

Figure 5

Orbitally resolved Ni $e_g$ and Ti $t_{2g}$ densities of states (red: Ni $d_{x^2-y^2}$; blue: Ni $d_{3z^2-r^2}$; green: Ti $d_{xy}$; purple: Ti $d_{xz}+d_{yz}$), computed at $U_{\mathrm{Ni}}=6\mathrm{eV}$ and $U_{\mathrm{Ti}}=4\mathrm{eV}$ for ferromagnetic ($F$), stripe antiferromagnetic ($S$) and checkerboard antiferromagnetic ($G$) orderings.