  In the two-dimensional electron gas (2DEG) emerging at the transition metal
oxide surface and interface, it has been pointed out that the Rashba spin-orbit
interaction, the momentum-dependent spin splitting due to broken inversion
symmetry and atomic spin-orbit coupling, can have profound effects on
electronic ordering in the spin, orbit, and charge channels, and may help give
rise to exotic phenomena such as ferromagnetism-superconductivity coexistence
and topological superconductivity. Although a large Rashba splitting is
expected to improve experimental accessibility of such phenomena, it has not
been understood how we can maximally enhance this splitting. Here, we present a
promising route to realize significant Rashba-type band splitting using a thin
film heterostructure. Based on first-principles methods and analytic model
analyses, a tantalate monolayer on BaHfO$_3$ is shown to host two-dimensional
bands originating from Ta $t_{2g}$ states with strong Rashba spin splittings -
up to nearly 10% of the bandwidth - at both the band minima and saddle points
due to the maximal breaking of the inversion symmetry. Such 2DEG band structure
makes this oxide heterostructure a promising platform for realizing both a
topological superconductor which hosts Majorana fermions and the electron
correlation physics with strong spin-orbit coupling.
