(a) Illustration of the conventional integer quantum Hall effect found, e.g., in 2D semiconductor systems. (b) Illustration of the unconventional half-integer quantum Hall effect found, e.g., in a graphene layer. (c) The experimentally observed Landau quantization of the topological surface states in for several magnetic field strengths. Curves are offset vertically for clarity. Note in particular the peak occurring at the Fermi level (around sample bias), which is independent of the field strength. This serves as a hallmark of the unconventional quantization form shown in (b). (d) STM picture of a surface in the presence of triangular-shaped defects (bright white spots). By selecting one point in such a map and plotting the curves as a function of bias voltage, one is able to probe the density of states similarly to (c).