Large Polarization Near 50 μC/cm² in a Single Unit Cell Layer SrTiO₃

The generally nonpolar SrTiO₃ has attracted more attention recently because of its possibly induced novel polar states and related paraelectric-ferroelectric phase transitions. By using controlled pulsed laser deposition, high-quality, ultrathin, and strained SrTiO₃ layers were obtained. Here, transmission electron microscopy and theoretical simulations have unveiled highly polar states in SrTiO₃ films even down to one unit cell at room temperature, which were stabilized in the PbTiO₃/SrTiO₃/PbTiO₃ sandwich structures by in-plane tensile strain and interfacial coupling, as evidenced by large tetragonality (∼1.05), notable polar ion displacement (0.019 nm), and thus ultrahigh spontaneous polarization (up to ∼50 μC/cm²). These values are nearly comparable to those of the strong ferroelectrics as the PbZrTi_1-O₃ family. Our findings provide an effective and practical approach for integrating large strain states into oxide films and inducing polarization in nonpolar materials, which may broaden the functionality of nonpolar oxides and pave the way for the discovery of new electronic materials.