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

The polarity discontinuity across LaAlO 3 /SrTiO 3 (LAO/STO) heterostructure induces
electronic reconstruction involving the formation of two-dimensional electron gas (2DEG).
The heterostructure also exhibits structural distortion in close correlation with the 2DEG
formation, which can be characterized by either antiferrodistortive (AFD) rotation,
ferroelectric (FE) distortion, or both. Here, by using density functional theory (DFT)
calculations, we show that the AFD and FE modes are cooperatively coupled in the (111)-
oriented LAO/STO heterostructure; they coexist below the critical thickness (t c ) and
disappear simultaneously above t c with the formation of 2DEG. Electron energy loss
spectroscopy simulation of the O-K edge provide direct evidence of oxygen vacancy (V O )
formation at the LAO (111) surface, which acts as the source of 2DEG. Tracing the AFD
rotation and FE distortion of LAO from the interface to the surface reveals that their
evolution is strongly correlated with the V O distribution. The present study demonstrates that
the AFD and FE modes in an oxide heterostructure emerge as a consequence of the intricate
interplay of the interface orientation, lattice misfit strain, and internal polar field; it further
demonstrates that their combination can drive competitive or cooperative coupling by tuning
the relative evolution of the two modes.