Enhanced Spin-to-Charge Conversion Through Interface Modulation Between Co and Bi_1-xSb_x

Materials with strong spin-orbit coupling (SOC), such as heavy metals (HMs) and topological materials (TMs), are highly valued for their ability to convert charge currents into spin currents and vice versa. The efficiency of spin-charge interconversion, defined as the spin Hall angle, is crucial for spintronic applications. TMs, due to the Edelstein effect, exhibit significantly larger spin Hall angles compared to HMs. Among these materials, Bi_1-xSb_x alloys stand out for their colossal spin Hall angle and superior conductivity. For instance, Bi_0.9Sb_0.1 has a spin Hall angle of 52 and conductivity of approximately 10⁵ Ω^-1m^-1. Recent studies using THz emission spectroscopy on Co/Bi_1-xSb_x heterostructures have shown that the topological surface state (TSS) of Bi_1-xSb_x is key in spin-to-charge conversion (SCC).<br/>Our study focuses on enhancing SCC efficiency by modulating the interface between Co and Bi_1-xSb_x. We systematically investigated the SCC and band properties when inserting HMs at this interface using THz emission spectroscopy and density functional theory (DFT) calculations. We found that inserting a bismuth 3 bilayer (3 BL) between Co and Bi_0.8Sb_0.2 increases THz emission by 171%, compared to Co/Bi_0.8Sb_0.2 heterostructures. Conversely, the Co/Bi/Bi_0.2Sb_0.8 structure showed a negligible increase in THz emission amplitude compared to Co/Bi_0.2Sb_0.8. DFT calculations and ultraviolet photoelectron spectroscopy revealed that the insertion of Bi creates a non-trivial and efficient band structure, enhancing SCC efficiency depending on x in Co/Bi/Bi_1-xSb_x.<br/>We also investigated the insertion of various HMs (e.g., Sb, Pt) between Co and Bi_1-xSb_x. Enhanced SCC efficiency was observed only in the Co/Bi(3 BL)/Bi_0.8Sb_0.2 structure, while other HMs decreased THz emission amplitude. However, varying the thickness of Sb in Co/Sb(t nm)/Bi_1-xSb_x showed that SCC efficiency could be optimized by leveraging spin Berry curvature at specific high symmetry points. This study highlights interface modulation as a promising strategy for designing efficient spintronic devices.