Top: Fermi contours with Rashba splitting, and non-equilibrium spin accumulation resulting from a shift of Fermi contours by spin injection. Bottom left: Experimental setup for the spin pumping measurement. Bottom right: Schematic of spin-to-charge current conversion at the Cu/Bi2O3 interface. Spin currents are generated by spin pumping from a Py layer in ferromagnetic resonance to a Cu layer and reach a Cu/Bi2O3 interface. Then, the spin-to-charge current conversion occurs owing to the spin-momentum locking at the Rashba interface.