ABSTRACT
The helical edge states of 2DTIs have higher-dimensional analogues,
Dirac-like 2D states at a surface of a bulk material. A bulk system
with the Dirac surface states inside the band gap is a three-
dimensional topological insulator (3DTI). To elucidate the origin of
the surface states, in this section we introduce a microscopic model
based on the Kane k · p Hamiltonian with a nontrivial boundary condition. We shall see that out of all bulk degrees of freedom
the surface state inherits only the electron spin. Similar to the
helical edge states, the spin of the surface carrier is locked to the
direction of its momentum. In the 2D surface geometry the spin-
momentum locking gives rise to a new transport phenomenon, weak
antilocalization. We shall discuss both a simple qualitative picture
and the detailed theory of this phenomenon.