Symmetry Lowering in Extreme-Electron-Density Perovskite Quantum Wells

Abstract

We report on structural distortions in extreme-electron density ($\ensuremath{\sim}6\ifmmode\times\else\texttimes\fi{}{10}^{14}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$) 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.