Spectroscopic Evidence for Competing Reconstructions in Polar Multilayers ${\mathrm{LaAlO}}_3/{\mathrm{LaVO}}_3/{\mathrm{LaAlO}}_3$

We have studied the valence redistribution of V in ${\mathrm{LaAlO}}_3/{\mathrm{LaVO}}_3/{\mathrm{LaAlO}}_3$ trilayers, which are composed of only polar layers grown on ${\mathrm{SrTiO}}_3$ (001) substrates, by core-level photoemission spectroscopy. We have found that the V valence is intermediate between ${\mathrm{V}}^{3+}$ and ${\mathrm{V}}^{4+}$ for thin ${\mathrm{LaAlO}}_3$ cap layers, decreases with increasing cap-layer thickness, and finally recovers the bulk value of ${\mathrm{V}}^{3+}$ at $\sim 10$ unit-cell thickness. In order to interpret these results, we propose that the atomic reconstruction of the polar ${\mathrm{LaAlO}}_3$ surface competes with the purely electronic V valence change so that the polar catastrophe is avoided at the cost of minimum energy.

Figure 1

Core-level spectra of the ${\mathrm{LaAlO}}_3/{\mathrm{LaVO}}_3/{\mathrm{LaAlO}}_3$ trilayer samples. (a) Schematic view of the ${\mathrm{LaAlO}}_3/{\mathrm{LaVO}}_3/{\mathrm{LaAlO}}_3$ trilayer samples grown on a ${\mathrm{SrTiO}}_3$ (001) substrate. The ${\mathrm{LaAlO}}_3$ cap-layer thickness was varied from 1 to 15 unit cells (uc) ($x=1,\cdots ,15\mathrm{uc}$). (b) O $1s$ and V $2p$ core-level spectra of the trilayer samples. (c) Intensity of the V $2p$ core level relative to that of the O $1s$ core level as a function of ${\mathrm{LaAlO}}_3$ cap-layer thickness. The experimental data were well fitted to the function $a\exp (-x/d)$ where $d=8.7\pm 1\mathrm{uc}$.

Figure 2

${\mathrm{LaAlO}}_3$ cap-layer thickness dependence of the V core-level spectra of the ${\mathrm{LaAlO}}_3(x\mathrm{uc})/{\mathrm{LaVO}}_3(3\mathrm{uc})/{\mathrm{LaAlO}}_3(30\mathrm{uc})$ trilayer samples grown on ${\mathrm{SrTiO}}_3$ substrates. (a) V $1s$ core-level spectra and their line-shape analyses. (b) V $2p_{3/2}$ core-level spectra and their line-shape analyses. (c) ${\mathrm{LaAlO}}_3$ cap-layer thickness dependence of the ${\mathrm{V}}^{3+}$ intensity relative to the total V intensity (${\mathrm{V}}^{3+}+{\mathrm{V}}^{4+}$) in the V $1s$ and $2p_{3/2}$ core-level spectra. The dashed curve is a guide to the eye.

Figure 3

Schematic illustrations of the reconstructions resolving the polar catastrophe in the all polar ${\mathrm{LaAlO}}_3/{\mathrm{LaVO}}_3/{\mathrm{LaAlO}}_3$ trilayers. (a) The structures with all polar planes suffer from a polar catastrophe. (b) The top surface of ${\mathrm{LaAlO}}_3$ undergoes the usual atomic reconstruction, whereas the bottom ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface accepts the extra electrons through the electronic reconstruction ${\mathrm{Ti}}^{4+}\to {\mathrm{Ti}}^{3.5+}$, thereby providing a compensating dipole. (c) Alternatively, the valence change (${\mathrm{V}}^{3+}\to {\mathrm{V}}^{3.5+}$) can also create extra electrons, resulting in purely electronic reconstructions on both sides of the trilayer. In the limit of a thick top ${\mathrm{LaAlO}}_3$ layer, process (b) overcomes process (c).

Figure 4

Typical Ti $1s$ core-level spectrum of the ${\mathrm{LaAlO}}_3(5\mathrm{uc})/{\mathrm{LaVO}}_3(3\mathrm{uc})/{\mathrm{LaAlO}}_3(30\mathrm{uc})$ trilayer grown on an ${\mathrm{SrTiO}}_3$ substrate.