Electronic Reconstruction at the Isopolar ${\mathrm{LaTiO}}_3/{\mathrm{LaFeO}}_3$ Interface: An X-Ray Photoemission and Density-Functional Theory Study

We report the formation of a nonmagnetic band insulator at the isopolar interface between the antiferromagnetic Mott-Hubbard insulator ${\mathrm{LaTiO}}_3$ and the antiferromagnetic charge transfer insulator ${\mathrm{LaFeO}}_3$. By density-functional theory calculations, we find that the formation of this interface state is driven by the combination of O band alignment and crystal field splitting energy of the $t_{2g}$ and $e_g$ bands. As a result of these two driving forces, the Fe $3d$ bands rearrange and electrons are transferred from Ti to Fe. This picture is supported by x-ray photoelectron spectroscopy, which confirms the rearrangement of the Fe $3d$ bands and reveals an unprecedented charge transfer up to $1.2\pm 0.2e^{-}/\text{interface}$ unit cell in our ${\mathrm{LaTiO}}_3/{\mathrm{LaFeO}}_3$ heterostructures.

Figure 1

Atomic and orbital projected DOS as well as schematic band structure of (a) bulk ${\mathrm{LaTiO}}_3$, (b) bulk ${\mathrm{LaFeO}}_3$, and (c)–(e) a ($1/1$) ${\mathrm{LaTiO}}_3/{\mathrm{LaFeO}}_3$ superlattice. Total states are marked in gray, O $p$ states in black, Fe and Ti $t_{2g}$ states in red, and Fe and Ti $e_g$ states in blue. The Fermi level is indicated by the dotted line.

Figure 2

(a) Sketch of the ${\mathrm{LaTiO}}_3/{\mathrm{LaFeO}}_3$ sample geometry. (b) A typical $1\times 1\mu \mathrm{m}$ AFM height image of a ${\mathrm{LaTiO}}_3/{\mathrm{LaFeO}}_3$ heterostructure. (c) Fe $2p$ XPS spectra of ${\mathrm{LaTiO}}_3/{\mathrm{LaFeO}}_3$ heterostructures for various thicknesses of ${\mathrm{LaFeO}}_3$, as well as of a 30 u.c. ${\mathrm{LaFeO}}_3$ film and a ($2/2$) ${\mathrm{LaAlO}}_3/{\mathrm{LaFeO}}_3$ heterostructure. The solid and open circles mark the ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{2+}$ peaks, respectively. (d) Valence band XPS spectra of ${\mathrm{LaTiO}}_3/{\mathrm{LaFeO}}_3$ heterostructures for various thicknesses of ${\mathrm{LaFeO}}_3$. All spectra were taken near normal emission ($\theta =3°$).

Figure 3

${\mathrm{Fe}}^{2+}$ spectral weight versus ${\mathrm{LaFeO}}_3$ thickness for both bulk (blue) and surface (red) sensitive XPS measurements, taking $[{\mathrm{Fe}}_{\text{total}}]=[{\mathrm{Fe}}^{3+}]+[{\mathrm{Fe}}^{2+}]=1$. The inset is a schematic view of the ${\mathrm{Fe}}^{2+}$ fraction [$p({\mathrm{Fe}}^{2+})$] across the ${\mathrm{LaFeO}}_3$ layer (indicated by the solid red curve). ${\mathrm{Fe}}^{2+}$ (${\mathrm{Fe}}^{3+}$) fraction is given in dark (light) blue. In addition, an indication of the XPS sensitivity for both surface (53°) and bulk (3°) sensitive measurements is shown. $z$ indicates the direction perpendicular to the surface.