Conversion between spin and charge currents in topological-insulator/nonmagnetic-metal systems

The charge current in a topological insulator (TI) will induce a spin accumulation (Edelstein effect or EE), from which the spin current will be generated. Inversely, the spin current injection into the TI will induce a charge current called the inverse Edelstein effect (IEE). Some experimental and theoretical works have been done for the understanding of either EE or IEE. However, little experimental work incorporating both processes in the same TI sample has been done. In this work, we propose a phenomenological model to understand EE and IEE in the TI-based system. Based on this model, efficiencies of EE and IEE can be directly derived, which is consistent with previous theoretical work based on Boltzmann transport theory and obeying the energy conservation law. We also measure EE and IEE efficiencies experimentally in a TI/Ru/CoFeB system by spin-torque ferromagnetic resonance and spin pumping, respectively. The experimental results are consistent with our model, which proves that the spin-charge conversion in TI can be understood in the framework of (I)EE instead of (inverse) spin Hall effect. By combining theories and experiments, we find that enhancing interfacial transparency is crucial for enhancing EE efficiency, and avoiding metallic contact is crucial for enhancing IEE efficiency.