  The electrical and structural characteristics of SmTiO3/SrTiO3/SmTiO3 and
GdTiO3/SrTiO3/GdTiO3 heterostructures are compared. Both types of structures
contain narrow SrTiO3 quantum wells, which accommodate a confined, high-density
electron gas. As shown previously [Phys. Rev. B 86, 201102(R) (2012)] SrTiO3
quantum wells embedded in GdTiO3 show a metal-to-insulator transition when
their thickness is reduced so that they contain only two SrO layers. In
contrast, quantum wells embedded in SmTiO3 remain metallic down to a single SrO
layer thickness. Symmetry-lowering structural distortions, measured by
quantifying the Sr-column displacements, are present in the insulating quantum
wells, but are either absent or very weak in all metallic quantum wells,
independent of whether they are embedded in SmTiO3 or in GdTiO3. We discuss the
role of orthorhombic distortions, orbital ordering, and strong electron
correlations in the transition to the insulating state.
