Atomic Scale Understanding of Ferroelectricity and Superconductivity in SrTiO3
The central material of interest for this thesis is strontium titanate (SrTiO3). Doped SrTiO3 exhibits a wide range of remarkable properties. The focus of this work is to investigate its ferroelectric and superconducting behavior. We utilize scanning transmission electron microscopy (STEM) to understand the role of the nanoscale SrTiO3 structure in these properties. SrTiO3 is an incipient ferroelectric in unstrained, pure form, but easily becomes ferroelectric when subjected to small perturbations. Understanding the nature of the ferroelectric transition in SrTiO3 is essential as it can play an important role in other properties. A hallmark of order-disorder transitions is the formation of polar domains above the Curie temperature. These polar regions percolate below the Curie temperature to form a long-range ordered ferroelectric state. Using high-angle annular dark-field STEM and analyzing off-centering of Ti columns, we show local polar regions at room temperature in compressively strained SrTiO3 films, highlighting the order-disorder nature of the ferroelectric transition in this material. Next, we focus on understanding the competition between mobile carriers and polar crystal distortions. Elucidating the nature of this competition is of great interest for polar superconductors, which have attracted significant interest for their potential to host unconventional superconducting states. We observe a systematic suppression of the nanodomains and ferroelectricity with increasing amount of Sm dopant atoms. The itinerant electrons screen polar distortions and disrupt the nanodomains above the Curie temperature. The results provide direct evidence that long-range Coulomb interactions, already present in the paraelectric phase, are driving the ferroelectric transition and are becoming increasingly short-ranged with doping. Moreover, we show that the disorder caused by the dopant atoms themselves presents a second contribution to the destabilization of the ferroelectric state. This disorder destroys the nanodomains at high dopant concentrations. The focus of the second part of this thesis concerns the superconductivity of SrTiO3. Doped SrTiO3 is a superconductor whose pairing mechanism is still not fully understood. Superconductivity in SrTiO3 occurs in the vicinity of a ferroelectric instability, and recent theories suggest a link between two orders. Here, by using atomic structure imaging, we find that the superconducting critical temperature correlates with the length scale of polar order. The superconducting transition temperature is enhanced when polar nanodomains are sufficiently large. In these cases, the Cooper pairs reside in a non-centrosymmetric environment with strong spin-orbit coupling. The findings point to the length scale of polar nanodomains and spin-orbit coupling as important parameters controlling the superconductivity of SrTiO3. We investigate the robustness of superconductivity to magnetic and nonmagnetic disorder and discuss the importance of findings in explaining the unconventional nature of superconductivity in this material.