Theory of quasiparticle interference on the surface of a strong topological insulator

Electrons on the surface of a strong topological insulator, such as Bi₂Te₃ or Bi_1−xSb_x, form a topologically protected helical liquid whose excitation spectrum contains an odd number of massless Dirac fermions. A theoretical survey and classification is given of the universal features, observable by the ordinary and spin-polarized scanning tunneling spectroscopy, in the interference patterns resulting from the quasiparticle scattering by magnetic and nonmagnetic impurities in such a helical liquid. Our results confirm the absence of backscattering from nonmagnetic impurities observed in recent experiments and predict new interference features, uniquely characteristic of the helical liquid, when the scatterers are magnetic.