Metal-insulator transition at the LaAlO${}_3$/SrTiO${}_3$ interface revisited: A hybrid functional study

We investigate the electronic properties of the ${\mathrm{LaAlO}}_3$/${\mathrm{SrTiO}}_3$ interface using density functional theory. In contrast to previous studies, which relied on (semi-)local functionals and the $\text{GGA}+U$ method, we here use a recently developed hybrid functional to determine the electronic structure. This approach offers the distinct advantage of accessing both the metallic and insulating multilayers on a parameter-free equal footing. As compared to calculations based on semilocal GGA functionals, our hybrid functional calculations lead to a considerably increased band gap for the insulating systems. The details of the electronic structure show substantial deviations from those obtained by GGA calculations. This casts severe doubts on all previous results based on semilocal functionals. In particular, corrections using rigid band shifts (“scissors operator”) cannot lead to valid results.

Figure 1

Initial idealized (left) and optimized (right) structures with $5\frac{1}{2}$ central layers of ${\mathrm{SrTiO}}_3$ sandwiched between two layers of ${\mathrm{LaAlO}}_3$ from each side.

Figure 2

Total DOSs for LAO thicknesses of two to six unit cells obtained in the GGA and hybrid functional calculations. The exact gap values are given, for the insulating systems.

Figure 3

Projected $t_{2g}$ and $e_g$ DOSs for the Ti1 atom.

Figure 4

Projected DOSs for the apical (O${}_a$, between Al and Ti at the interface) and basal (O${}_b$, bonded to Al only) O atoms.