Large Polarization Near 50 μC/cm 2 in a Single Unit Cell Layer SrTiO 3 .

The generally nonpolar SrTiO ₃ 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 SrTiO ₃ layers were obtained. Here, transmission electron microscopy and theoretical simulations have unveiled highly polar states in SrTiO ₃ films even down to one unit cell at room temperature, which were stabilized in the PbTiO ₃ /SrTiO ₃ /PbTiO ₃ 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/cm ² ). These values are nearly comparable to those of the strong ferroelectrics as the PbZr _x Ti _{1- x} O ₃ 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.