Non-collinear and asymmetric polar moments at back-gated SrTiO₃ interfaces

The mechanism of the gate-field-induced metal-to-insulator transition of the electrons at the interface of SrTiO₃ with LaAlO₃ or AlO_x is of great current interest. Here, we show with infrared ellipsometry and confocal Raman spectroscopy that an important role is played by a polar lattice distortion that is non-collinear, highly asymmetric and hysteretic with respect to the gate field. The anomalous behavior and the large lateral component of the underlying local electric field is explained in terms of the interplay between the oxygen vacancies, that tend to migrate and form extended clusters at the antiferrodistortive domain boundaries, and the interfacial electrons, which get trapped/detrapped at the oxygen vacancy clusters under a positive/negative gate bias. Our findings open new perspectives for the defect engineering of lateral devices with strongly enhanced and hysteretic local electric fields that can be manipulated with various parameters, like strain, temperature, or photons.