Independence of Antiferrodistortive and Ferroelectric Transition in Strained SrTiO₃ Revealed by STEM

Doped SrTiO₃ is a prominent example of an unconventional superconductor that cannot be described by the BCS-Eliashberg paradigm. One of the main open questions is how superconductivity is connected to two other order parameters in this material, namely an antiferrodistortive instability and a ferroelectric instability. Recently, we have revealed the presence of static polar distortions in the paraelectric phase of SrTiO₃, and have discussed their role in the superconducting transition.<br/>Some studies have suggested that the antiferrodistortive distortion plays no role in the superconductivity, while others speculate that antiferrodistortive domain walls may enhance superconductivity. In this study, we use scanning transmission electron microscopy (STEM) to unveil the antiferrodistortive and ferroelectric distortion in the compressively strained, doped SrTiO₃ films. We identified the antiferrodistortive phase in real space by mapping out oxygen atoms using annular bright field (ABF) STEM. The dopant effects on the antiferrodistortive and ferroelectric phase are further elucidated by applying a free energy model with near density functional theory-level accuracy. The antiferrodistortive phase exhibits a single-domain structure, as expected due to the compressive in-plane strain. Using maps of the Ti column positions, we simultaneously measured the local polar domains. We find that unlike the local polar ferroelectric phase, which is suppressed by dopants, antiferrodistortive order persists in the presence of dopants. We show that the two transitions are largely independent of each other and the antiferrodistortive phase plays no role in the superconductivity.