Role of locally polar regions in the superconductivity of SrTiO3

Understanding the interaction between polar and superconducting order parameters may hold the key to several classes of superconductors that remain poorly understood, including SrTiO3 and several tellurides. Here we show that doped, strained SrTiO3 films can exhibit both global or local polar order, respectively, depending on the amount of epitaxial mismatch strain, thereby providing a platform to understand how inversion symmetry breaking affects superconductivity. We find that the superconducting critical temperature correlates with the length scale of polar order. In particular, the transition temperature is enhanced when polar nanodomains are sufficiently large or, in the extreme limit, films are globally ferroelectric. In these cases, the Cooper pairs reside in a noncentrosymmetric environment. Conversely, low transition temperatures are found when the nanodomains are small. The findings point to the length scale of polar nanodomains and spin-orbit coupling as important parameters controlling the superconductivity of SrTiO3. The ability to control the size of the polar domains opens up new opportunities to design and control the nature of superconductivity in a wide range of materials.