Cooperative evolution of polar distortion and nonpolar rotation of oxygen octahedra in oxide heterostructures

Polarity discontinuity across LaAlO3/SrTiO3 (LAO/STO) heterostructures induces electronic reconstruction involving the formation of two-dimensional electron gas (2DEG) and structural distortions characterized by antiferrodistortive (AFD) rotation and ferroelectric (FE) distortion. We show that AFD and FE modes are cooperatively coupled in LAO/STO (111) heterostructures; they coexist below the critical thickness (t(c)) and disappear simultaneously above tc with the formation of 2DEG. Electron energy-loss spectroscopy and density functional theory (DFT) calculations provide direct evidence of oxygen vacancy (VO) formation at the LAO (111) surface, which acts as the source of 2DEG. Tracing the AFD rotation and FE distortion of LAO reveals that their evolution is strongly correlated with VO distribution. The present study demonstrates that AFD and FE modes in oxide heterostructures emerge as a consequence of interplay between misfit strain and polar field, and further that their combination can be tuned to competitive or cooperative coupling by changing the interface orientation.

Automated summary

Polarity discontinuity in LaAlO3/SrTiO3 (LAO/STO) heterostructures induces the formation of two-dimensional electron gas (2DEG) and structural distortions involving antiferrodistortive (AFD) rotation and ferroelectric (FE) distortion. AFD and FE modes are cooperatively coupled below the critical thickness in LAO/STO (111) heterostructures, disappearing simultaneously above the critical thickness with the formation of 2DEG. Oxygen vacancy (VO) formation at the LAO (111) surface acts as the source of 2DEG, with the evolution of AFD rotation and FE distortion strongly correlated with VO distribution. This study demonstrates the emergence of AFD and FE modes in oxide heterostructures as a result of the interplay between misfit strain and polar field, with their coupling being tunable by changing the interface orientation.