Symmetry Lowering in Extreme-Electron-Density Perovskite Quantum Wells

We report on structural distortions in extreme-electron density ($\sim 6\times {10}^{14}{\mathrm{cm}}^{-2}$) confined quantum wells of ${\mathrm{SrTiO}}_3$ embedded in ${\mathrm{GdTiO}}_3$. Sr-column displacements are measured using high-angle annular dark-field imaging in scanning transmission electron microscopy. Using thick ${\mathrm{SrTiO}}_3$ layers as a reference, orthorhombic-like Sr-site displacements are observed when ${\mathrm{SrTiO}}_3$ quantum wells are thinner than 4 SrO layers, in precise agreement with an experimentally observed metal-to-insulator transition. It is shown that the structural displacements are absent in thicker quantum wells, even in the immediate proximity to the orthorhombic ${\mathrm{GdTiO}}_3$. The results are discussed in the context of strong electron correlations in this system.

Figure 1

(a) Schematic of the sample. ${\mathrm{SrTiO}}_3$ layer thicknesses are indicated on the right (in units of SrO layers). (b) HAADF image of 2 SrO layers embedded in ${\mathrm{GdTiO}}_3$ showing Sr column displacements. The red line is a guide to the eye. (c) HAADF image of the first ${\mathrm{SrTiO}}_3\mathrm{\text{-buffer}}/{\mathrm{GdTiO}}_3$ interface showing Gd displacements parallel to the growth direction. The ${\mathrm{GdTiO}}_3$ appears bright due to the larger atomic number of Gd. The angle $\theta$ (solid lines) is measured between three successive $A$-site cations. The first SrO row shows no Sr displacements (dashed line). (d) Lower magnification HAADF STEM image of the sample.

Figure 2

(a) $(110{)}_o\Vert (001{)}_c$ orientation of orthorhombic ${\mathrm{GdTiO}}_3$ cell (solid black outline) on cubic LSAT (blue shaded). Projections are shown along $[110{]}_o$ (left) and $[001{]}_o$ (right). Both occur within one cross-section sample due to domain formation. (b) Projected views of ${\mathrm{GdTiO}}_3$ along $[110{]}_o$ (top) and $[001{]}_o$ (bottom), respectively, along with simulated and experimental PACBED patterns from these projections. The contrast in experimental patterns was adjusted to match background intensities of simulated patterns.

Figure 3

Measured deviation angle, $180°-\theta$, of each $A\mathrm{O}$ layer in the sample. The error bars represent the standard error of the mean.

Figure 4

(a) HAADF STEM images of 8 SrO, 4 SrO and 2 SrO quantum wells. (b) FT of the center 2 SrO layers ($64\times 1024$ pixel region) from each quantum well and (c) integrated intensity profiles of the FT. Extra peaks appear in the FT from the 2 SrO quantum well, indicated by arrows, which are absent in thicker layers.