Engineering nanoscale superconductivity through the competition between ferromagnetism and superconductivity

As electronic devices continue to decrease in size, it become increasingly more important to understand how to engineer nanoscale systems. Reduced dimensionality in superconductors allows fluctuations to play a greater role in the behavior of the system. For example, bulk iron selenide (FeSe) has a superconducting critical temperature of 8K, but the single layer FeSe grown on SrTiO₃ exhibits a significantly higher critical temperature. Motivated by these results, we have performed density functional theory (DFT) calculations for various systems of SrTiO₃ with different surface reconstructions for oxygen vacancies. We find that a robust surface ferromagnetism can emerge due to the periodic pattern of oxygen vacancies in the presence of Hubbard on-site interaction. Using a single-band model for the superconductor-ferromagnet heterostructure, we demonstrate that for nanoscale systems the superconducting properties are highly susceptible to modulation caused by the ferromagnetism, which can lead to the decrease of the pairing and critical temperature.