Unexpectedly robust protection from backscattering in the topological insulator Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$

Electron scattering in the topological surface state (TSS) of the bulk-insulating topological insulator Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ was studied using quasiparticle interference observed by scanning tunneling microscopy. It was found that not only the 180$^{\circ}$ backscattering but also a wide range of backscattering angles of 100$^{\circ}$--180$^{\circ}$ is effectively prohibited in the TSS. This conclusion was obtained by comparing the observed scattering vectors with the diameters of the constant-energy contours of the TSS, which were measured for both occupied and unoccupied states using time- and angle-resolved photoemission spectroscopy. The unexpectedly robust protection from backscattering in the TSS is a good news for applications, but it poses a challenge to the theoretical understanding of the transport in the TSS.