Symmetry lowering in extreme-electron-density perovskite quantum wells.

We report on structural distortions in extreme-electron density (∼6×10(14)  cm(-2)) confined quantum wells of SrTiO3 embedded in GdTiO3. Sr-column displacements are measured using high-angle annular dark-field imaging in scanning transmission electron microscopy. Using thick SrTiO3 layers as a reference, orthorhombic-like Sr-site displacements are observed when SrTiO3 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 GdTiO3. The results are discussed in the context of strong electron correlations in this system.