UC Santa Barbara

We report on structural distortions in extreme-electron density (6x10¹⁴ 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.