of the cubic (octahedral) case.](fig2.pdf){width="45.00000%"}

In this study, we have theoretically constructed a realistic oxide heterostructure that has a surface 2DEG with a strong Rashba spin-orbit interaction. Our idea is to consider a 5$d$ TM oxide monolayer on a substrate, where 2DEG predominantly lies in the outermost monolayer film, maximizing the effect of the broken inversion symmetry from the substrate. Specifically, we attempt to replace the outer layers of perovskite oxide (001) surface with another perovskite thin film layers, in which the electronic bands of the substrate need to lie sufficiently far from the conduction band minimum (CBM) to make all essential low-energy physics originate from the thin film states near CBM. After calculations of a number of candidate perovskite oxides for the heterostructure, we find that TaO$_2$/KO or TaO$_2$/BaO layer on BaHfO$_3$ (001) surface (Fig. \[fig:atomicstr\]) is a promising candidate structure possessing Ta $t_{2g}$-originated two-dimensional (2D) bands with a strong Rashba spin-orbit interaction. BaHfO$_3$ (BHO) is suitable as a substrate because its lattice structure (cubic at room temperature) matches with that of KTO, the only stable perovskite material containing TaO$_2$ [-@Zhurova2000; -@Maekawa2006], and the alignment of its conduction bands and the Ta $t_{2g}$ bands enables minimal hybridization. It should 