Surface termination effect of $\mathrm{SrTi}{O}_3$ substrate on ultrathin $\mathrm{SrRu}{O}_3$

A uniform 1-unit-cell-high step on the $\mathrm{SrTi}{O}_3$ (STO) substrate is a prerequisite for growing high-quality epitaxial oxide heterostructures. However, it is inevitable that defects induced by mixed substrate-surface termination exist at the interface, significantly impacting the properties of ultrathin films. In this study, we microscopically identify the origin for the lateral inhomogeneity in the growth of ultrathin $\mathrm{SrRu}{O}_3$ films due to the step effects of $\mathrm{SrTi}{O}_3$ (001). By using atomic-resolved scanning transmission electron microscopy, we observe two distinct types of step propagation along the [011] and $\left[0\overline{1}1\right]$ crystallographic direction in $\mathrm{SrTi}{O}_3-\mathrm{SrRu}{O}_3$ heterostructures, respectively. In particular, the type-II $\left[0\overline{1}1\right]$ step results in lateral discontinuity of monolayer $\mathrm{SrRu}{O}_3$ and originates from the SrO-terminated regions along the $\mathrm{Ti}{O}_2-\mathrm{terminated}$ step edge. Such an induced lateral discontinuity should be responsible for the distinct electronic and magnetic properties of monolayer $\mathrm{SrRu}{O}_3$. Our findings underscore the critical importance of using single-termination STO substrate to achieve high-quality termination-selective films and to unveil the intrinsic properties of epitaxial films in the atomic limit.