The metal-to-insulator transition in ${\mathrm{BaTiO}}_3$ is investigated using electrostatic doping, which avoids effects from disorder and strain that would accompany chemical doping. ${\mathrm{SmTiO}}_3/{\mathrm{BaTiO}}_3/{\mathrm{SrTiO}}_3$ heterostructures are doped with a constant sheet carrier density of $3\times {10}^{14}{\mathrm{cm}}^{-2}$ that is introduced via the polar ${\mathrm{SmTiO}}_3/{\mathrm{BaTiO}}_3$ interface. Below a critical ${\mathrm{BaTiO}}_3$ thickness, the structures exhibit metallic behavior with high carrier mobilities at low temperatures, similar to ${\mathrm{SmTiO}}_3/{\mathrm{SrTiO}}_3$ interfaces. Above this thickness, data indicate that the ${\mathrm{BaTiO}}_3$ layer becomes ferroelectric. The ${\mathrm{BaTiO}}_3$ lattice parameters increase to a value consistent with a strained, tetragonal unit cell, the structures are insulating below $\sim 125\mathrm{K}$, and the mobility drops by more than an order of magnitude, indicating self-trapping of carriers. The results shed light on the interplay between charge carriers and ferroelectricity.

• Received 3 March 2016

DOI:https://doi.org/10.1103/PhysRevLett.117.037602