Topological Surface-Dominated Spintronic THz Emission in Topologically Nontrivial Bi_1−xSb_x Films

Topological materials have significant potential for spintronic applications owing to their superior spin–charge interconversion. Here, the spin-to-charge conversion (SCC) characteristics of epitaxial Bi_1−xSb_x films is investigated across the topological phase transition by spintronic terahertz (THz) spectroscopy. An unexpected, intense spintronic THz emission is observed in the topologically nontrivial semimetal Bi_1−xSb_x films, significantly greater than that of Pt and Bi₂Se₃, which indicates the potential of Bi_1−xSb_x for spintronic applications. More importantly, the topological surface state (TSS) is observed to significantly contribute to SCC, despite the coexistence of the bulk state, which is possible via a unique ultrafast SCC process, considering the decay process of the spin-polarized hot electrons. This means that topological material-based spintronic devices should be fabricated in a manner that fully utilizes the TSS, not the bulk state, to maximize their performance. The results not only provide a clue for identifying the source of the giant spin Hall angle of Bi_1−xSb_x, but also expand the application potential of topological materials by indicating that the optically induced spin current provides a unique method for focused-spin injection into the TSS.