Spin-polarized surface bands of a three-dimensional topological insulator studied by high-resolution spin- and angle-resolved photoemission spectroscopy

The spin-polarized surface band structure of the three-dimensional (3D) quantum spin Hall phase of Bi_1−xSb_x (x=0.12–0.13) was studied by spin- and angle-resolved photoemission spectroscopy (SARPES) using a high-yield spin polarimeter equipped with a high-resolution electron spectrometer. The spin-integrated spectra were also measured and compared to those of Bi_1−xSb_x with x=0.04. Band dispersions of the edge states were fully elucidated between the two time-reversal-invariant points, and , of the (111) surface Brillouin zone. The observed spin-polarized band dispersions at x=0.12–0.13 indicate an odd number of the band crossing at the Fermi energy, giving unambiguous evidence that this system is a 3D strong topological insulator, and determine the 'mirror chirality' to be −1, which excludes the existence of a Dirac point in the middle of the – line. The present research demonstrates that the SARPES measurement with energy resolution ⩽50 meV is one of the critical techniques for complementing the topological band theory for spins and spin currents.