  Two-dimensional topological insulators (2D TIs) are a highly desired quantum
phase but few materials have demonstrated clear signatures of a 2D TI state. It
has been predicted that 2D TIs can be created from thin films of
three-dimensional TIs by reducing the film thickness until the surface states
hybridize. Here, we employ this technique to report the first observation of a
2D TI state in epitaxial thin films of cadmium arsenide, a prototype Dirac
semimetal in bulk form and a 3D TI in thin films. Using magnetotransport
measurements with electrostatic gating, we observe a Landau level spectrum and
quantum Hall effect that are in excellent agreement with those of an ideal 2D
TI. Specifically, we observe a crossing of the zeroth Landau levels at a
critical magnetic field. We show that the film thickness can be used to tune
the critical magnetic field. Moreover, a larger change in film thickness causes
a transition from a 2D TI to a 2D trivial insulator, just as predicted by
theory. The high degree of tunability available in epitaxial cadmium arsenide
heterostructures can thus be used to fine-tune the 2D TI, which is essential
for future topological devices.
