One of the most fundamental phenomena and a reminder of the electrons relativistic nature is the Rashba spin splitting for broken inversion symmetry. Usually this splitting is a tiny relativistic correction, hardly discernible in experiment. Interfacing a ferroelectric, BaTiO$_3$, and a heavy 5$d$ metal with a large spin-orbit coupling, Ba(Os,Ir)O$_3$, we show that giant Rashba spin splittings are indeed possible and even fully controllable by an external electric field. Based on density functional theory and a microscopic tight binding understanding, we conclude that the electric field is amplified and stored as a ferroelectric Ti-O distortion which, through the network of oxygen octahedra, also induces a large Os-O distortion. The BaTiO$_3$/BaOsO$_3$ heterostructure is hence the ideal test station for studying the fundamentals of the Rashba effect. It allows intriguing application such as the Datta-Das transistor to operate at room temperature.
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