Preparation of SrTiO3 bicrystal substrates with atomic-level controlled boundaries for Josephson junction fabrication

Grain-boundary Josephson junctions formed by epitaxial growth of high-temperature (${T}_{\mathrm{c}}$) superconductor films on bicrystal substrates have been employed to determine the pairing symmetry of high-${T}_{\mathrm{c}}$ superconductivity. Since high-${T}_{\mathrm{c}}$ superconductivity predominately takes place within a single unit cell along the $c$ axis and superconducting coherence length is typically in the nanometer scale, the microstructure of grain boundaries of the bicrystal substrates is crucial for fabricating junctions required for Josephson tunneling study. In this study, we develop a two-step method to prepare insulating $\mathrm{SrTi}{\mathrm{O}}_{3}(001)$ bicrystal substrates with atomically uniform and coalesced grain boundaries. Using in situ scanning tunneling microscopy, we demonstrate that FeSe films grown on such substrates exhibit single-facet grain boundaries, leading to ideal Josephson junctions for investigating the pairing symmetry of FeSe superconductors.