While the prevailing early belief was that topological surface states in insulators are insensitive to non-magnetic disorder, and are destroyed (or at least gapped) by magnetic impurities,  the currently emerging consensus is that these statements are at best approximate. I will review the results showing that both localized and extended non-magnetic impurities can modify the topological state properties, and use those to motivate a study of a magnetic line defect on the surface of a topological Insulator.

I will show that such an extended defect generically supports two branches of spin-polarized and current carrying one-dimensional bound states. The velocity, and hence spin texture, of each of those branches can be independently tuned by a magnetic field rotated in the plane of the surface. I compute the local spin-resolved density of states as well as the net spin polarization and current due to both bound and scattering states as a function of potential and magnetic components of the scattering potential, and show that it varies nearly stepwise. This work lays the foundation for defect engineering of spin textures and macroscopic currents at topological insulator surfaces.