Non-collinear and asymmetric polar moments at back-gated SrTiO<inf>3</inf> interfaces

The mechanism of the gate-field-induced metal-to-insulator transition of the electrons at the interface of SrTiO3 with LaAlO3 or AlOx 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.F.L. and C.B. acknowledge enlightening discussions with S. Das, J. Maier, R. Merkle, A. Dubroka, and B. I. Shklovskii. Work at the University of Fribourg was supported by the Schweizerische Nationalfonds (SNF) by Grant No. 200020-172611. M.B. acknowledges support from the ERC Advanced grant n° 833973 “FRESCO” and the QUANTERA project “QUANTOX”. G.H. acknowledges financial support from Spanish Ministry of Science and Innovation (MCIN/AEI/10.13039/501100011033) through the Severo Ochoa FUNFUTURE (CEX2019-000917-S) and Grant No. PID2020-118479RB-I00, and Generalitat de Catalunya (2017 400 SGR 1377).With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe