Photoemission from Buried Interfaces in ${\mathrm{SrTiO}}_3/{\mathrm{LaTiO}}_3$ Superlattices

We have measured photoemission spectra of ${\mathrm{SrTiO}}_3/{\mathrm{LaTiO}}_3$ superlattices with a topmost ${\mathrm{SrTiO}}_3$ layer of variable thickness. A finite coherent spectral weight with a clear Fermi cutoff was observed at chemically abrupt ${\mathrm{SrTiO}}_3/{\mathrm{LaTiO}}_3$ interfaces, indicating that an “electronic reconstruction” occurs at the interface between the Mott insulator ${\mathrm{LaTiO}}_3$ and the band insulator ${\mathrm{SrTiO}}_3$. For ${\mathrm{SrTiO}}_3/{\mathrm{LaTiO}}_3$ interfaces annealed at high temperatures ($\sim 1000°\mathrm{C}$), which leads to $\mathrm{Sr}/\mathrm{La}$ atomic interdiffusion and hence to the formation of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{TiO}}_3$-like material, the intensity of the incoherent part was found to be dramatically reduced whereas the coherent part with a sharp Fermi cutoff was enhanced due to the spread of charge. These important experimental features are well reproduced by layer dynamical-mean-field-theory calculation.

Figure 1

Schematic views of the ${\mathrm{SrTiO}}_3/{\mathrm{LaTiO}}_3$ superlattice samples: capping layer only sample A; 1 uc capping sample B; 2 uc capping sample C; 2 uc capping and annealing sample D. Growth temperatures are also indicated.

Figure 2

UPS spectra of the ${\mathrm{SrTiO}}_3/{\mathrm{LaTiO}}_3$ superlattice samples: the capping layer only sample A, the 1 uc capping sample B, the 2 uc capping sample C, and the 2 uc capping and annealing sample D. (a) Valence-band spectra over a wide energy range. (b) Near Fermi-level spectra after background subtraction (see text).

Figure 3

Ti $2p\to 3d$ resonant PES spectra taken at 466 eV of the ${\mathrm{SrTiO}}_3/{\mathrm{LaTiO}}_3$ superlattice samples; the 1 uc capping sample B, the 2 uc capping sample C, and the 2 uc capping and annealing sample D. For comparison, off-resonant PES spectra taken at 600 eV are also shown.

Figure 4

DMFT calculation for STO-capped STO/LTO superlattices. (a) Model for the calculation. The number of STO-cap layers (denoted by $n_{\mathrm{out}}$) and the ratio of $\mathrm{La}/\mathrm{Sr}$ mixing are varied. (b) Simulated PES spectra taking into account the effect of finite photoelectron mean-free path of three unit cells corresponding to the resonant PES. For $n_{\mathrm{out}}=2$ and 3, the effect of 40% La atom diffusion is shown as broken curves.