Giant Bulk-Rashba Splitting in Polar Topological Insulator BiSbTeS$_2$

Bulk-Rashba spin splitting is forbidden in tetradymite topological insulators like Bi$_2$Se$_3$ or Bi$_2$Te$_3$, since their quintuple-layer stacking preserves inversion symmetry. We show that BiSbTeSe$_2$ escapes this restriction: in the Se-Bi-Se-Sb-Te sequence, the structure loses its inversion center, reducing the point group symmetry at $\Gamma$ from $D_{3d}$ to $C_{3v}$. First-principles density functional calculations with spin-orbit coupling show that this ordered structure retains bulk band inversion and a linearly dispersive surface state of a strong topological insulator. Additionally, its bulk bands acquire a pronounced linear-in-$k$ spin splitting away from $\Gamma$. Fitting the conduction- and valence-band doublets to symmetry-constrained two-band $k\cdot p$ Hamiltonians, we extract intrinsic linear Rashba coefficients of $\alpha_{\mathrm{CB}}\approx2.66~\mathrm{eV\,\text{\AA}}$ and $\alpha_{\mathrm{VB}}\approx0.35~\mathrm{eV\,\text{\AA}}$. The conduction-band value places ordered BiSbTeSe$_2$ among the strongest bulk-Rashba topological-insulator systems reported to date and approaches the coupling found in the benchmark polar Rashba semiconductor BiTeI.Sublattice ordering thus provides a route to giant bulk spin--momentum locking that coexists with protected topological surface states, offering a platform in which bulk Rashba and topological surface contributions to spin and charge transport can be investigated within the same material.