A topological insulator is bulk-insulating but hosts metallic surface states protected by time-reversal symmetry. These states form a Dirac cone — a conical band crossing at the Γ point where the Fermi level intersects.
The surface electrons are spin-momentum locked: the spin direction is perpendicular to the momentum, winding once around the Fermi surface (Z₂ invariant ν=1).
Backscattering is forbidden by topology — impurities cannot scatter electrons backward without flipping spin.