Direct mapping of the spin-filtered surface bands of a three-dimensional quantum spin Hall insulator

Spin-polarized band structure of the three-dimensional quantum spin Hall insulator Bi_1-xSbₓ $(x=0.12--0.13)$ was fully elucidated by spin-polarized angle-resolved photoemission spectroscopy using a high-yield spin polarimeter equipped with a high-resolution electron spectrometer. Between the two time-reversal-invariant points, Γ and M̄, of the (111) surface Brillouin zone, a spin-up band (Σ₁^' band) was found to cross the Fermi energy only once, providing unambiguous evidence for the strong topological insulator phase. The observed spin-polarized band dispersions determine the ``mirror chirality'' to be -1, which agrees with the theoretical prediction based on first-principles calculations.