The surfaces of a three-dimensional topological insulator each host a single Dirac fermion,
which, in a strong magnetic field, contribute to the transverse conductance in integer-and-a-
half multiples of the conductance quantum. The direct observation of this extra half integer,
the hallmark of the two-dimensional Dirac state, is usually thwarted by the fermion doubling
theorem—top and bottom surfaces are not measured independently. Here, we employ a
Corbino measurement geometry in which a current, induced by an ac magnetic field, is …