Colossal Spin Hall Conductivity in Charge-Massive Topologically Nontrivial Semimetal Phase of Bi _{1– <i>x</i>} Sb _<i>x</i> (012)

Topologically nontrivial semimetals (TSMs) possess spin-momentum locked topological surface states (TSS) with high charge conductivity (σ), offering a platform for low-power spintronic applications. However, its spin Hall conductivity (σ_SH), another important factor for spintronic applications, remains largely unexplored. Here, we demonstrate that Bi_1-<i>x</i>Sb_<i>x</i> in the TSM phase (<i>x</i> = 0.6-0.8), when grown along the distinct (012) orientation, exhibits a colossal σ_SH, far exceeding that of Pt. The σ of Bi_1-<i>x</i>Sb_<i>x</i> (012) in the TSM phase is also 1 to 2 orders of magnitude higher than that of conventional topological materials. We reveal a crystal orientation-selective activation of TSS in charge-to-spin conversion (CSC) of Bi_1-<i>x</i>Sb_<i>x</i> (012), in contrast to Bi_1-<i>x</i>Sb_<i>x</i> (001), where CSC is bulk-dominated. Our density functional theory calculations reveal multiple Dirac cones on the (012) surface, providing a compelling explanation for the observed colossal σ_SH via a two-channel model that incorporates both bulk and surface contributions. Our findings suggest TSMs, particularly Bi_1-<i>x</i>Sb_<i>x</i> (012), as an effective material system for next-generation low-power spintronic devices.