Hard x-ray photoemission study of $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3∕\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ multilayers

We have studied the electronic structure of multilayers composed of a band insulator $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ (LAO) and a Mott insulator $\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ (LVO) by means of hard x-ray photoemission spectroscopy, which has a probing depth as large as $\sim 60\mathrm{\AA }$. The Mott-Hubbard gap of LVO remained open at the interface, indicating that the interface is insulating unlike the $\mathrm{La}\mathrm{Ti}{\mathrm{O}}_3∕\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ multilayers. We found that the valence of V in LVO was partially converted from ${\mathrm{V}}^{3+}$ to ${\mathrm{V}}^{4+}$ only at the interface on the top side of the LVO layer and that the amount of ${\mathrm{V}}^{4+}$ increased with LVO layer thickness. We suggest that the electronic reconstruction to eliminate the polarity catastrophe inherent in the polar heterostructure is the origin of the highly asymmetric valence change at the LAO/LVO interfaces.

Figure 1

Schematic view of the $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3∕\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ multilayer samples. Sample $A$: $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ (3 uc)/$\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ (3 uc)/$\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ (30 uc)/$\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$. Sample $B$: $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ (3 uc)/$\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ (50 uc)/$\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$. Sample $C$: $\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ (50 uc)/$\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$.

Figure 2

Valence-band photoemission spectra of the $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3∕\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ multilayer samples. (a) Valence-band spectra over a wide energy range. (b) V $3d$ band region.

Figure 3

V $1s$ core-level photoemission spectra of the $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3∕\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ multilayer samples.

Figure 4

O $1s$ and V $2p$ core-level photoemission spectra of the $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3∕\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ multilayer samples. (a) shows wide energy region and (b) is an enlarged plot of the V $2p_{3∕2}$ spectra.

Figure 5

Fitting results for the V $1s$ and $2p_{3∕2}$ core-level spectra. (a) V $1s$ core level of sample $A$ ($\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ 3 uc), (b) V $2p_{3∕2}$ core level of sample $A$ ($\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ 3 uc), (c) V $1s$ core level of sample $B$ ($\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ 50 uc), and (d) V $2p_{3∕2}$ core level of sample $B$ ($\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ 50 uc).

Figure 6

Ratio of ${\mathrm{V}}^{4+}$ determined under various experimental conditions using hard x rays and soft x rays (Ref. 9). (a) Sample $A$ (3 uc $\mathrm{La}\mathrm{V}{\mathrm{O}}_3$) and (b) sample $B$ (50 uc $\mathrm{La}\mathrm{V}{\mathrm{O}}_3$). Here, SX is a result of soft x-ray photoemission and HX is of hard x-ray photoemission. In the case of SX, the values in the parenthesis denote the values of ${\theta }_e$.

Figure 7

Models for the V valence distributions in the $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3∕\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ multilayer samples. $A$: $\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ 3 uc. $A1$ is an asymmetric model, whereas $A2$ is a symmetric model. $B$: $\mathrm{La}\mathrm{V}{\mathrm{O}}_3$ 50 uc.